PC-based conferencing system

ABSTRACT

A video subsystem resides partially on a host processor and partially on a video board. Audio and communications subsystems reside partially on the host processor and partially on a audio/communications board. The video subsystem generates and passes local compressed video signals corresponding to local analog video signals to the communications subsystem. The audio subsystem receives and generates local compressed audio signals corresponding to local analog audio signals to the communications subsystem. The communications subsystem transmits the local compressed video and audio signals over a communications link to a remote computer system. The communications subsystem also receives remote compressed video and audio signals over the communications link from the remote computer system, and passes the remote compressed video and audio signals to the video and audio subsystems, respectively. The video subsystem generates remote analog video signals corresponding to the remote compressed video signals for local playback. The audio subsystem generates remote analog audio signals corresponding to the remote compressed audio signals for local playback.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to audio/video conferencing, and, inparticular, to systems for real-time audio, video, and data conferencingin windowed environments on personal computer systems.

2. Description of the Related Art

It is desirable to provide real-time audio, video, and data conferencingbetween personal computer (PC) systems operating in windowedenvironments such as those provided by versions of Microsoft® Windowsoperating system. There are difficulties, however, with providingreal-time conferencing in non-real-time windowed environments.

It is accordingly an object of this invention to overcome thedisadvantages and drawbacks of the known art and to provide real-timeaudio, video, and data conferencing between PC systems operating innon-real-time windowed environments.

It is a particular object of the present invention to provide real-timeaudio, video, and data conferencing between PC systems operating under aMicrosoft® Windows operating system.

Further objects and advantages of this invention will become apparentfrom the detailed description of a preferred embodiment which follows.

SUMMARY OF THE INVENTION

A video subsystem resides partially on a host processor and partially ona video board. Audio and communications subsystems reside partially onthe host processor and partially on a audio/communications board. Thevideo subsystem generates and passes local compressed video signalscorresponding to local analog video signals to the communicationssubsystem. The audio subsystem receives and generates local compressedaudio signals corresponding to local analog audio signals to thecommunications subsystem. The communications subsystem transmits thelocal compressed video and audio signals over a communications link to aremote computer system. The communications subsystem also receivesremote compressed video and audio signals over the communications linkfrom the remote computer system, and passes the remote compressed videoand audio signals to the video and audio subsystems, respectively. Thevideo subsystem generates remote analog video signals corresponding tothe remote compressed video signals for local playback. The audiosubsystem generates remote analog audio signals corresponding to theremote compressed audio signals for local playback.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description ofthe preferred embodiment, the appended claims, and the accompanyingdrawings in which:

FIG. 1 is a block diagram representing real-time point-to-point audio,video, and data conferencing between two PC systems, according to apreferred embodiment of the present invention;

FIG. 2 is a block diagram of the hardware configuration of theconferencing system of each PC system of FIG. 1;

FIG. 3 is a block diagram of the hardware configuration of the videoboard of the conferencing system of FIG. 2;

FIG. 4 is a block diagram of the hardware configuration of theaudio/comm board of the conferencing system of FIG. 2;

FIG. 5 is a block diagram of the software configuration of theconferencing system of each PC system of FIG. 1;

FIG. 6 is a block diagram of a preferred embodiment of the hardwareconfiguration of the audio/comm board of FIG. 4;

FIG. 7 is a block diagram of the conferencing interface layer betweenthe conferencing applications of FIG. 5, on one side, and the comm,video, and audio managers of FIG. 5, on the other side;

FIG. 8 is a representation of the conferencing call finite state machine(FSM) for a conferencing session between a local conferencing system(i.e., caller) and a remote conferencing system (i.e., callee);

FIG. 9 is a representation of the conferencing stream FSM for eachconferencing system participating in a conferencing session;

FIG. 10 is a representation of the video FSM for the local video streamand the remote video stream of a conferencing system during aconferencing session;

FIG. 11 is a block diagram of the software components of the videomanager of the conferencing system of FIG. 5;

FIG. 12 is a representation of a sequence of N walking key frames;

FIG. 13 is a representation of the audio FSM for the local audio streamand the remote audio stream of a conferencing system during aconferencing session;

FIG. 14 is a block diagram of the architecture of the audio subsystem ofthe conferencing system of FIG. 5;

FIG. 15 is a block diagram of the interface between the audio task ofFIG. 5 and the audio hardware of audio/comm board of FIG. 2;

FIG. 16 is a block diagram of the interface between the audio task andthe comm task of FIG. 5;

FIG. 17 is a block diagram of the comm subsystem of the conferencingsystem of FIG. 5;

FIG. 18 is a block diagram of the comm subsystem architecture for twoconferencing systems of FIG. 5 participating in a conferencing session;

FIG. 19 is a representation of the comm subsystem application FSM for aconferencing session between a local site and a remote site;

FIG. 20 is a representation of the comm subsystem connection FSM for aconferencing session between a local site and a remote site;

FIG. 21 is a representation of the comm subsystem control channelhandshake FSM for a conferencing session between a local site and aremote site;

FIG. 22 is a representation of the comm subsystem channel establishmentFSM for a conferencing session between a local site and a remote site;

FIG. 23 is a representation of the comm subsystem processing for atypical conferencing session between a caller and a callee;

FIG. 24 is a representation of the structure of a video packet as sentto or received from the comm subsystem of the conferencing system ofFIG. 5;

FIG. 25 is a representation of the compressed video bitstream for theconferencing system of FIG. 5;

FIG. 26 is a representation of a compressed audio packet for theconferencing system of FIG. 5;

FIG. 27 is a representation of the reliable transport comm packetstructure;

FIG. 28 is a representation of the unreliable transport comm packetstructure;

FIG. 29 are diagrams indicating typical connection setup and teardownsequences;

FIGS. 30 and 31 are diagrams of the architecture of the audio/commboard; and

FIG. 32 is a diagram of the audio/comm board environment.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Point-To-Point Conferencing Network

Referring now to FIG. 1, there is shown a block diagram representingreal-time point-to-point audio, video, and data conferencing between twoPC systems, according to a preferred embodiment of the presentinvention. Each PC system has a conferencing system 100, a camera 102, amicrophone 104, a monitor 106, and a speaker 108. The conferencingsystems communicate via an integrated services digital network (ISDN)110. Each conferencing system 100 receives, digitizes, and compressesthe analog video signals generated by camera 102 and the analog audiosignals generated by microphone 104. The compressed digital video andaudio signals are transmitted to the other conferencing system via ISDN110, where they are decompressed and converted for play on monitor 106and speaker 108, respectively. In addition, each conferencing system 100may generate and transmit data signals to the other conferencing system100 for play on monitor 106. In a preferred embodiment, the video anddata signals are displayed in different windows on monitor 106. Eachconferencing system 100 may also display the locally generated videosignals in a separate window.

Camera 102 may be any suitable camera for generating NSTC or PAL analogvideo signals. Microphone 104 may be any suitable microphone forgenerating analog audio signals. Monitor 106 may be any suitable monitorfor displaying video and graphics images and is preferably a VGAmonitor. Speaker 108 may be any suitable device for playing analog audiosignals and is preferably a headset.

Conferencing System Hardware Configuration

Referring now to FIG. 2, there is shown a block diagram of the hardwareconfiguration of each conferencing system 100 of FIG. 1, according to apreferred embodiment of the present invention. Each conferencing system100 comprises host processor 202, video board 204, audio/comm board 206,and ISA bus 208.

Referring now to FIG. 3, there is shown a block diagram of the hardwareconfiguration of video board 204 of FIG. 2, according to a preferredembodiment of the present invention. Video board 204 comprises industrystandard architecture (ISA) bus interface 310, video bus 312, pixelprocessor 302, video random access memory (VRAM) device 304, videocapture module 306, and video analog-to-digital (A/D) converter 308.

Referring now to FIG. 4, there is shown a block diagram of the hardwareconfiguration of audio/comm board 206 of FIG. 2, according to apreferred embodiment of the present invention. Audio/comm board 206comprises ISDN interface 402, memory 404, digital signal processor (DSP)406, and ISA bus interface 408, audio input/output (I/O) hardware 410.

Conferencing System Software Configuration

Referring now to FIG. 5, there is shown a block diagram of the softwareconfiguration each conferencing system 100 of FIG. 1, according to apreferred embodiment of the present invention. Video microcode 530resides and runs on pixel processor 302 of video board 204 of FIG. 3.Comm task 540 and audio task 538 reside and run on DSP 406 of audio/commboard 206 of FIG. 4. All of the other software modules depicted in FIG.5 reside and run on host processor 202 of FIG. 2.

Video, Audio, and Data Processing

Referring now to FIGS. 3, 4, and 5, audio/video conferencing application502 running on host processor 202 provides the top-level local controlof audio and video conferencing between a local conferencing system(i.e., local site or endpoint) and a remote conferencing system (i.e.,remote site or endpoint). Audio/video conferencing application 502controls local audio and video processing and establishes links with theremote site for transmitting and receiving audio and video over theISDN. Similarly, data conferencing application 504, also running on hostprocessor 202, provides the top-level local control of data conferencingbetween the local and remote sites. Conferencing applications 502 and504 communicate with the audio, video, and comm subsystems usingconferencing application programming interface (API) 506, video API 508,comm API 510, and audio API 512. The functions of conferencingapplications 502 and 504 and the APIs they use are described in furtherdetail later in this specification.

During conferencing, audio I/O hardware 410 of audio/comm board 206digitizes analog audio signals received from microphone 104 and storesthe resulting uncompressed digital audio to memory 404 via ISA businterface 408. Audio task 538, running on DSP 406, controls thecompression of the uncompressed audio and stores the resultingcompressed audio back to memory 404. Comm task 540, also running on DSP406, then formats the compressed audio format for ISDN transmission andtransmits the compressed ISDN-formatted audio to ISDN interface 402 fortransmission to the remote site over ISDN 110.

ISDN interface 402 also receives from ISDN 110 compressed ISDN-formattedaudio generated by the remote site and stores the compressedISDN-formatted audio to memory 404. Comm task 540 then reconstructs thecompressed audio format and stores the compressed audio back to memory404. Audio task 538 controls the decomposition of the compressed audioand stores the resulting decompressed audio back to memory 404. ISA businterface then transmits the decompressed audio to audio I/O hardware410, which digital-to-analog (D/A) converts the decompressed audio andtransmits the resulting analog audio signals to speaker 108 for play.

Thus, audio capture/compression and decompression/playback arepreferably performed entirely within audio/comm board 206 without goingthrough the host processor. As a result, audio is preferablycontinuously played during a conferencing session regardless of whatother applications are running on host processor 202.

Concurrent with the audio processing, video A/D converter 308 of videoboard 204 digitizes analog video signals received from camera 102 andtransmits the resulting digitized video to video capture module 306.Video capture module 306 decodes the digitized video into YUV colorcomponents and delivers uncompressed digital video bitmaps to VRAM 304via video bus 312. Video microcode 530, running on pixel processor 302,compresses the uncompressed video bitmaps and stores the resultingcompressed video back to VRAM 304. ISA bus interface 310 then transmitsvia ISA bus 208 the compressed video to host interface 526 running onhost processor 202.

Host interface 526 passes the compressed video to video manager 516 viavideo capture driver 522. Video manager 516 calls audio manager 520using audio API 512 for synchronization information. Video manager 516then time-stamps the video for synchronization with the audio. Videomanager 516 passes the time-stamped compressed video to communications(comm) manager 518 using comm application programming interface (API)510. Comm manager 518 passes the compressed video through digital signalprocessing (DSP) interface 528 to ISA bus interface 408 of audio/commboard 206, which stores the compressed video to memory 404. Comm task540 then formats the compressed video for ISDN transmission andtransmits the ISDN-formatted compressed video to ISDN interface 402 fortransmission to the remote site over ISDN 110.

ISDN interface 402 also receives from ISDN 110 ISDN-formatted compressedvideo generated by the remote site system and stores the ISDN-formattedcompressed video to memory 404. Comm task 540 reconstructs thecompressed video format and stores the resulting compressed video backto memory 404. ISA bus interface then transmits the compressed video tocomm manager 518 via ISA bus 208 and DSP interface 528. Comm manager 518passes the compressed video to video manager 516 using comm API 510.Video manager 516 decompresses the compressed video and transmits thedecompressed video to the graphics device interface (GDI) (not shown) ofMicrosoft® Windows for eventual display in a video window on monitor106.

For data conferencing, concurrent with audio and video conferencing,data conferencing application 504 generates and passes data to commmanager 518 using conferencing API 506 and comm API 510. Comm manager518 passes the data through board DSP interface 532 to ISA bus interface408, which stores the data to memory 404. Comm task 540 formats the datafor ISDN transmission and stores the ISDN-formatted data back to memory404. ISDN interface 402 then transmits the ISDN-formatted data to theremote site over ISDN 110.

ISDN interface 402 also receives from ISDN 110 ISDN-formatted datagenerated by the remote site and stores the ISDN-formatted data tomemory 404. Comm task 540 reconstructs the data format and stores theresulting data back to memory 404. ISA bus interface 408 then transmitsthe data to comm manager 518, via ISA bus 208 and DSP interface 528.Comm manager 518 passes the data to data conferencing application 504using comm API 510 and conferencing API 506. Data conferencingapplication 504 processes the data and transmits the processed data toMicrosoft® Windows GDI (not shown) for display in a data window onmonitor 106.

Preferred Hardware Configuration for Conferencing System

Referring again to FIG. 2, host processor 202 may be any suitablegeneral-purpose processor and is preferably an Intel® processor such asan Intel®486 microprocessor. Host processor 202 preferably has at least8 megabytes of host memory. Bus 208 may be any suitable digitalcommunications bus and is preferably an Industry Standard Architecture(ISA) PC bus. Referring again to FIG. 3, video A/D converter 308 ofvideo board 204 may be any standard hardware for digitizing and decodinganalog video signals that are preferably NTSC or PAL standard videosignals. Video capture module 306 may be any suitable device forcapturing digital video color component bitmaps and is preferably anIntel® ActionMedia® II Capture Module. Video capture module 306preferably captures video as subsampled 4:1:1 YUV bitmaps (i.e., YUV9 orYVU9). Memory 304 may be any suitable computer memory device for storingdata during video processing such as a random access memory (RAM) deviceand is preferably a video RAM (VRAM) device with at least 1 megabyte ofdata storage capacity. Pixel processor 302 may be any suitable processorfor compressing video data and is preferably an Intel® pixel processorsuch as an Intel® i750® Pixel Processor. Video bus 312 may be anysuitable digital communications bus and is preferably an Intel® DVI®bus. ISA bus interface 310 may be any suitable interface between ISA bus208 and video bus 312, and preferably comprises three Intel®ActionMedia® Gate Arrays and ISA configuration jumpers.

Referring now to FIG. 6, there is shown a block diagram of a preferredembodiment of the hardware configuration of audio/comm board 206 of FIG.4. This preferred embodiment comprises:

Two 4-wire S-bus RJ-45 ISDN interface connectors, one for output to ISDN110 and one for input from ISDN 110. Part of ISDN interface 402 of FIG.4.

Standard bypass relay allowing incoming calls to be redirected to adown-line ISDN phone (not shown) in case conferencing system power isoff or conferencing software is not loaded. Part of ISDN interface 402.

Two standard analog isolation and filter circuits for interfacing withISDN 110. Part of ISDN interface 402.

Two Siemens 8-bit D-channel PEB2085 ISDN interface chips. Part of ISDNinterface 402.

Texas Instruments (TI) 32-bit 33 MHz 320c31 Digital Signal Processor.Equivalent to DSP 406.

Custom ISDN/DSP interface application specified integrated circuit(ASIC) to provide interface between 8-bit Siemens chip set and 32-bit TIDSP. Part of ISDN interface 402.

256 Kw Dynamic RAM (DRAM) memory device. Part of memory 404.

32 Kw Static RAM (SRAM) memory device. Part of memory 404.

Custom DSP/ISA interface ASIC to provide interface between 32-bit TI DSPand ISA bus 208. Part of ISA bus interface 408.

Serial EEPROM to provide software jumpers for DSP/ISA interface. Part ofISA bus interface 408.

Audio Codec 4215 by Analog Devices, Inc. for sampling audio in formatsuch as ADPCM, DPCM, or PCM format. Part of audio I/O hardware 410.

Analog circuitry to drive audio I/O with internal speaker for playbackand audio jacks for input of analog audio from microphone 104 and foroutput of analog audio to speaker 108. Part of audio I/O hardware 410.

Referring now to FIGS. 30 and 31, there are shown diagrams of thearchitecture of the audio/comm board. The audio/comm board consistsbasically of a slave ISA interface, a TMS320C31 DSP core, an ISDN BRI Sinterface, and a high quality audio interface.

The C31 Interface is a 32-bit non-multiplexed data port to the VC ASIC.It is designed to operate with a 27-33 Mhz C31. The C31 address isdecoded for the ASIC to live between 400 000H and 44F FFFH. All accessesto local ASIC registers (including the FIFO's) are 0 wait-state.Accesses to the I/O bus (locations 440 000H through 44F FFFH) have 3wait states inserted. Some of the registers in the ASIC are 8 and 16bits wide. In these cases, the data is aligned to the bottom (bit 0 andup) of the C31 data word. The remainder of the bits will be read as a"0". All non-existent or reserved register locations will read as a "0".

The B-channel interfaces provide a 32-bit data path to and from the B1and B2 ISDN data channels. They are FIFO buffered to reduce interruptoverhead and latency requirements. The Line-side and Phone-sideinterfaces both support transparent data transfer--used for normalphone-call,1 FAX, modem and H.221 formatted data. Both interfaces alsosupport HDLC formatting of the B data per channel to support V.120 "datadata" transfer.

The receive and transmit FIFO's are 2 words deep, a word being 32 bitswide (C31 native data width). Full, half and empty indications for allFIFO's are provided in the B-channel status registers. Note that thepolarity of these indications vary between receive and transmit. This isto provide the correct interrupt signaling for interrupt synchronizeddata transfer.

The transparent mode sends data received in the B-channel transmitFIFO's to the SSI interface of the ISACs. The transmitted data is notformatted in any way other than maintaining byte alignment (i.e., bits0, 8, 16, 24 of the FIFO data are always transmitted in bit 0 of theB-channel data). The written FIFO data is transmitted byte 0 first, byte3 last--where byte 0 is bits 0 through 7, and bit 0 is sent first.

Transparent mode received data is also byte aligned to the incomingB-channel data stream and assembled as byte 0, byte 1, byte 2, byte 3.Receive data is written into the receive FIFO after all four types havearrived.

The ISAC I/O Interface provides an 8 bit multiplexed data bus used toaccess the Siemens PEB2085s (ISAC). The 8 bits of I/O address come frombits 0 through 7 of the C31 address. Reads and writes to this interfaceadd 3 wait-states to the C31 access cycle. Buffered writes are notsupported in this version of the ASIC.

Each ISAC is mapped directly into its own 64 byte address space (6 validbits of address). Accesses to the ISAC are 8 bits wide and are locatedat bit positions 0 to 7 in the C31 32 bit word. Bits 8 through 23 arereturned as "0"s on reads.

The PB2085's provide the D-channel access using this interface.

The Accelerator Module Interface is a high bandwidth serialcommunication path between the C31 and another processor which will beused to add MIPs to the board. Certain future requirements such as g.728audio compression will require the extra processing power.

The data transfers are 32 bit words sent serially at about 1.5 Mbits/s.The VC ASIC buffers these transfers with FICOs which are 2 words deep toreduce interrupt overhead and response time requirements. The statusregister provide flags for FIFO full, half, empty and over/under-run(you should never get an under-run). Any of these can be used asinterrupt sources as selected in the Serial Port Mask register.

The following paragraphs describe the ISA interface of the audio/commboard. The ISA interface is the gate array that provides an interfacebetween the multi-function board and the ISA bus. Further, the ASIC willcontrol background tasks between a DSP, SAC, and Analog Phone lineinterfaces. The technology chosen for the ASIC is the 1 micron CMOS-6family from NEC.

Referring now to FIG. 32, there is shown a diagram of the audio/commboard environment. The following is a description of the signal groups.

    __________________________________________________________________________    ISA Bus Signals                                                               AEN           The address enable signal is used to de-gated the CPU and                     other                                                                         devices from the bus during DMA cycles. When this signal is                   active                                                                        (high) the DMA controller has control of the bus. The ASIC                    does not                                                                      respond to bus cycles when AEN is active.                       IOCS16#       The I/O 16-bit chip select is used by 16-bit I/O devices to                   indicate that                                                                 it can accommodate a 16-bit transfer. This signal is                          decoded off of                                                                address only.                                                   IOW#          This is an active low signal indicating the an I/O write                      cycle is                                                                      being performed.                                                IOR#          This is an active low signal indicating the an I/O read                       cycle is being                                                                performed.                                                      IRQ3, IRQ4,   These signals are interrupt requests. An interrupt request                    is generated                                                    IRQ5, IRQ9,   when an IRQ is raised from a low to a high. The IRQ must                      remain                                                          IRQ10, IRA11, high until the interrupt service routine acknowledges the                     interrupt.                                                      IRQ15                                                                         RESET         This signal is used to initialize system logic upon power                     on.                                                             SBHE#         The system bus high enable signal indicates that data                         should be driven                                                              onto the upper byte of the 16-bit data bus.                     SA(9:0)       These are the system address lines used to decode I/O                         address space                                                                 used by the board. This scheme is compatible with the ISA                     bus.                                                                          These addresses are valid during the entire command cycle.      SD(15:0)      These are the system data bus lines.                            DSP Signals                                                                   H1CLK         H1CLK is the DSP primary bus clock. All events in the                         primary bus                                                                   are referenced to this clock. The frequency of this clock                     is half the                                                                   frequency of the clock driving the DSP. See the TMS320C31                     data                                                                          manual chapter 13.                                              D(31:0)       These are the DSP 32-bit data bus. Data lines 16, 17, and                     18 also                                                                       interface to the EEPROM. Note that the DSP must be in reset                   and the                                                                       data bus tri-stated before access to the EEPROM. This date                    bus also                                                                      supplies the board ID when the read while the DSP is reset                    (see                                                                          HAUTOID register).                                              C31.sub.-- RST#                                                                             This is the DSP active low reset signal.                        A23-A0        These DSP address lines are used to decode the address                        space by the                                                                  ASIC.                                                           R/W#          This signal indicates whether the current DSP external                        access is a read                                                              (high) or a write (low)                                         STRB#         This is an active low signal form the DSP indicating that                     the current                                                                   cycle is to the primary bus.                                    RDY#          This signal indicates that the current cycle being                            performed on the                                                              primary bus of the DSP can be completed.                        HOLD#         The Hold signal is an active low signal used to request the                   DSP                                                                           relinquish control of the primary bus. Once the hold has                      been                                                                          acknowledge all address, data and status lines are                            tri-stated until Hold                                                         is released. This signal will be used to implement the DMA                    and                                                                           DRAM Refresh.                                                   HOLDA#        This is the Hold Acknowledge signal which is the active low                   indication                                                                    that the DSP has relinquished control of the bus.               INT2#         This C31 interrupt is used by the ASIC for DMA and Command                    interrupts.                                                     INTE1#        Interrupt the C31 on COM Port events.                           INT0#         Analog Phone Interrupts.                                        Memory Signals                                                                MEMWR1# and   These signals are active low write strobes for memory banks                   1 and 2.                                                        MEMWR2#                                                                       B1OE#,        These signals are active low output enables for memory                        banks 1 and 2.                                                  B20E#                                                                         SR.sub.-- CS# This is a active low chip selected for the SRAM that makes                    up bank2.                                                       CAS#          This the active low column address strobe to the DRAM.          RAS#          This the active low row address strobe to the DRAM.             H1D12,        These signals are a 12 and 24 nS delay of the H1CLK.            H1D24                                                                         MUX           Mux is the signal that controls the external DRAM address                     mux.                                                                          When this signal is low the CAS addresses are selected and                    when it is                                                                    high the RAS addresses are selected.                            EEPROM Signals                                                                EESK          This is the EEPROM clock signal. This signal is multiplexed                   with the                                                                      DSP data signal 1D16. This signal can only be valid while                     the DSP is                                                                    in reset.                                                       EEDI          This is the input data signal to the EEPROM. This signal                      is                                                                            multiplexed with the DSP data signal D17. This signal can                     only be                                                                       valid while the DSP is in reset.                                EEDO          This is the data output of the EEPROM. This signal is                         multiplexed with the DSP data signal D18. This signal can                     only                                                                          be valid while the DSP is in reset.                             EECS          This is the chip select signal for the EEPROM. This signal                    is NOT                                                                        multiplexed and can only be drive active (HIGH) during DSP                    reset.                                                          Stereo Audio Codec (SAC)                                                      SP.sub.-- DC  This signal controls the SAC mode of operation. When this                     signal is                                                                     high the SAC is in data or master mode. When this signal is                   lw the                                                                        SAC is in control or slave mode.                                SP.sub.-- SCLK                                                                              This is the Soundport clock input signal. This clock will                     either                                                                        originate from the Soundport or the ASIC.                       SP.sub.-- SDIN                                                                              This serial data input from the Soundport. The data here is                   shifted in                                                                    on the falling edge of the SP.sub.-- CLK.                       SP.sub.-- SDOUT                                                                             This is the serial data output signal for the Soundport.                      The data is                                                                   shifted out on the rising edge of the SP.sub.-- CLK.            SP.sub.-- FSYNC                                                                             This is the frame synchronization signal for the Soundport.                   This signal                                                                   will originate from the ASIC when the Soundport is in slave                   mode or                                                                       the Soundport is being programmed in control mode. When                       the                                                                           Soundport is in master mode the frame sync will originate                     from the                                                                      Soundport and will have a frequency equal to the sample                       rate.                                                           CODEC Signals                                                                 24.576MHZ     This clock signal is used to derive clocks used within the                    ASIC and the                                                                  2.048MHz CODEC clock.                                           COD.sub.-- FS1,                                                                             These signals are the CODEC frame syncs, each signal                          correspond to                                                   COD.sub.-- FS2,                                                                             one of the four CODECs.                                         DOC.sub.-- FS3,                                                               COD.sub.-- FS4                                                                COD.sub.-- SDOUT                                                                            This signal is the serial data output signal of the CODES.      COD.sub.-- SDIN                                                                             This signal is the serial data input signal to the CODECs.      COD.sub.-- SCLK                                                                             This a 2.048MHz clock used to clock data in and out of the                    four                                                                          CODECs. The serial data is clocked out on the rising edge                     and in on                                                                     the falling edge.                                               Analog Phone Signals                                                          LPSENSL1      Line1 off hook loop current sense. If this signal is low                      and                                                                           BYPSRLY1 is high it indicates the Set 1 has gone off hook.                    If the                                                                        signal is low and the BYPSRLY1 is low it indicates that the                   board has                                                                     gone off hook. This signal is not latched and therefore is                    a Real-time-                                                                  signal.                                                         LPSENSPH1     Set 1 off hook loop current sense. If this signal is low it                   indicates the                                                                 Set 1 has gone off hook. This can only take place when                        BYPSRLY1 is                                                                   low. This signal is not latched and therefore is a                            Real-time-signal.                                               LPSENSL2      Line2 off hook loop current sense. If this signal is low                      and                                                                           BYPSRLY2 is high it indicates the Set 1 has gone off hook.                    If the                                                                        signal is low and the BYPSRLY2 is low it indicates that the                   board has                                                                     gone off hook. This signal is not latched and therefore is                    a Real-time-                                                                  signal.                                                         LPSENSPH2     Set 2 off hook loop current sense. If this signal is low it                   indicates the                                                                 Set 1 has gone off hook. This can only take place when                        BYPSRLY2 is                                                                   low. This signals is not latched and therefore is a                           Real-time-signal.                                               RINGDETL1     Line 1 Ring Detect. If this input signal is low the Line                      is                                                                            ringing.                                                        RINGDETL2     Line 2 Ring Detect. If this input signal is low the Line                      is                                                                            ringing.                                                        CALLDETL2     Call Detect for Line 1. This signal is cleared low by                         software                                                                      to detect 1200 baud FSK data between the first and second                     rings.                                                          CALLDETL2     Call Detect for Line 2. This signal is cleared low by                         software                                                                      to detect 1200 baud FSK data between the first and second                     rings.                                                          PDOHL1        Pulse Dial Off hook for Line 1. This signal is pulsed to                      dial phone                                                                    numbers on pulse dial systems. It is also used to take the                    line off hook                                                                 when low.                                                       PDOHL2        Pulse Dial Off hook for Line 2. This signal is pulsed to                      dial phone                                                                    numbers on pulse dial systems. It is also used to take the                    line off hook                                                                 when low.                                                       BYPSRLY1 and 2                                                                              This is an active low output signal controlling the Bypass                    Relay output.                                                                 When high the board is by-passed and the Line (1 or 2) is                     connected                                                                     the desk Set (1 or 2).                                          LOOPDIS                                                                       SWCLR#                                                                        Miscellaneous Signals                                                         6.144MHZ      This a 6.144 MHz clock signal used to drive the module that                   can                                                                           attached to the board. The module will then use this signal                   to                                                                            synthesize any frequency it requires.                           TEST1, TEST2, These are four test pins used by the ASIC designers two                       decrease ASIC                                                   TEST3, TEST4  manufacturing test vectors. The TEST2 pin is the output of                    the nand-                                                                     tree used by ATE.                                               VVD, VSS                                                                      __________________________________________________________________________

Those skilled in the art will understand that the present invention maycomprise configurations of audio/comm board 206 other than the preferredconfiguration of FIG. 6.

Software Architecture for Conferencing System

The software architecture of conferencing system 100 shown in FIGS. 2and 5 has three layers of abstraction. A computer supportedcollaboration (CSC) infrastructure layer comprises the hardware (i.e.,video board 204 and audio/comm board 206) and host/board driver software(i.e., host interface 526 and DSP interface 528) to support video,audio, and comm, as well as the encode method for video (running onvideo board 204) and encode/decode methods for audio (running onaudio/comm board 206). The capabilities of the CSC infrastructure areprovided to the upper layer as a device driver interface (DDI).

A CSC system software layer provides services for instantiating andcontrolling the video and audio streams, synchronizing the two streams,and establishing and gracefully ending a call and associatedcommunication channels. This functionality is provided in an applicationprogramming interface (API). This API comprises the extended audio andvideo interfaces and the communications APIs (i.e., conferencing API506, video API 508, video manager 516, video capture driver 522, commAPI 510, comm manager 518, Wave API 514, Wave driver 524, audio API 512,and audio manager 520).

A CSC applications layer brings CSC to the desktop. The CSC applicationsmay include video annotation to video mail, video answering machine,audio/video/data conferencing (i.e., audio/video conferencingapplication 502 and data conferencing application 504), and groupdecision support systems.

Audio/video conferencing application 502 and data conferencingapplication 504 rely on conferencing API 506, which in turn relies uponvideo API 508, comm API 510, and audio API 512 to interface with videomanager 516, comm manager 518, and audio manager 520, respectively. CommAPI 510 and comm manager 518 provide a transport-independent interface(TII) that provides communications services to conferencing applications502 and 504. The communications software of conferencing system 100supports different transport mechanisms, such as ISDN (e.g., V.120interface), SW56 (e.g., BATP's Telephone API), and LAN (e.g., SPX/IPX,TCP/IP, or NetBIOS). The TII isolates the conferencing applications fromthe underlying transport layer (i.e., transport-medium-specific DSPinterface 528). The TII hides the network/connectivity specificoperations. In conferencing system 100, the TII hides the ISDN layer.The DSP interface 528 is hidden in the datalink module (DLM). The TIIprovides services to the conferencing applications for openingcommunication channels (within the same session) and dynamicallymanaging the bandwidth. The bandwidth is managed through thetransmission priority scheme.

In a preferred embodiment in which conferencing system 100 performssoftware video decoding, AVI capture driver 522 is implemented on top ofhost interface 526 (the video driver). In an alternative preferredembodiment in which conferencing system 100 performs hardware videodecoding, an AVI display driver is also implemented on top of hostinterface 526.

The software architecture of conferencing system 100 comprises threemajor subsystems: video, audio, and communication. The audio and videosubsystems are decoupled and treated as "data types" (similar to text orgraphics) with conventional operations like open, save, edit, anddisplay. The video and audio services are available to the applicationsthrough video-management and audio-management extended interfaces,respectively.

Audio/Video Conferencing Application

Audio/video conferencing application 502 implements the conferencinguser interface. Conferencing application 502 is implemented as aMicrosoft® Windows 3.1 application. One child window will display thelocal video image and a second child window will display the remotevideo image. Audio/video conferencing application 502 provides thefollowing services to conferencing system 100:

Manage main message loop.

Perform initialization and registers classes.

Handle menus.

Process toolbar messages.

Handles preferences.

Handles speed dial setup and selections.

Connect and hang up.

Handles handset window

Handle remote video.

Handle remote video window.

Handle local video.

Handle local video window.

Data Conferencing Application

Data conferencing application 504 implements the data conferencing userinterface. Data conferencing application is implemented as a Microsoft®Windows 3.1 application. The data conferencing application uses a"shared notebook" metaphor. The shared notebook lets the user copy afile from the computer into the notebook and review it with a remoteuser during a call. When the user is sharing the notebook (this time iscalled a "meeting"), the users see the same information on theircomputers, users can review it together, and make notes directly intothe notebook. A copy of the original file is placed in the notebook, sothe original remains unchanged. The notes users make during the meetingare saved with the copy in a meeting file. The shared notebook lookslike a notebook or stack of paper. Conference participants have accessto the same pages. Either participant can create a new page and fill itwith information or make notes on an existing page.

Conferencing API

Conferencing API 506 of FIG. 5 facilitates the easy implementation ofconferencing applications 502 and 504. Conferencing API 506 of FIG. 5provides a generic conferencing interface between conferencingapplications 502 and 504 and the video, comm, and audio subsystems.Conferencing API 506 provides a high-level abstraction of the servicesthat individual subsystems (i.e., video, audio, and comm) support. Themajor services include:

Making, accepting, and hanging-up calls.

Establishing and terminating multiple communication channels forindividual subsystems.

Instantiating and controlling local video and audio.

Sending video and audio to a remote site through the network.

Receiving, displaying, and controlling the remote video and audiostreams.

Conferencing applications 502 and 504 can access these services throughthe high-level conferencing API 506 without worrying about thecomplexities of low-level interfaces supported in the individualsubsystems.

In addition, conferencing API 506 facilitates the integration ofindividual software components. It minimizes the interactions betweenconferencing applications 502 and 504 and the video, audio, and commsubsystems. This allows the individual software components to bedeveloped and tested independent of each other. Conferencing API 506serves as an integration point that glues different software componentstogether. Conferencing API 506 facilitates the portability ofaudio/video conferencing application 502.

Conferencing API 506 is implemented as a Microsoft Windows Dynamic LinkLibrary (DLL). Conferencing API 506 translates the function calls fromconferencing application 502 to the more complicated calls to theindividual subsystems (i.e., video, audio, and comm). The subsystem calllayers (i.e., video API 508, comm API 510, and audio API 512) are alsoimplemented in DLLs. As a result, the programming of conferencing API506 is simplified in that conferencing API 506 does not need toimplement more complicated schemes, such as dynamic data exchange (DDE),to interface with other application threads that implement the servicesfor individual subsystems. For example, the video subsystem will usewindow threads to transmit/receive streams of video to/from the network.

Conferencing API 506 is the central control point for supportingcommunication channel management (i.e., establishing, terminatingchannels) for video and audio subsystems. Audio/video conferencingapplication 502 is responsible for supporting communication channelmanagement for the data conferencing streams.

Referring now to FIG. 7, there is shown a block diagram of theconferencing interface layer 700 between conferencing applications 502and 504 of FIG. 5, on one side, and comm manager 518, video manager 516,and audio manager 520, on the other side, according to a preferredembodiment of the present invention. Conferencing API 506 of FIG. 5comprises conferencing primitive validator 704, conferencing primitivedispatcher 708, conferencing callback 706, and conferencing finite statemachine (FSM) 702 of conferencing interface layer 700 of FIG. 7. CommAPI 510 of FIG. 5 comprises comm primitive 712 and comm callback 710 ofFIG. 7. Video API 508 of FIG. 5 comprises video primitive 716 of FIG. 7.Audio API 512 of FIG. 5 comprises audio primitive 720 of FIG. 7.

Conferencing primitive validator 704 validates the syntax (e.g., checksthe conferencing call state, channel state, and the stream state withthe conferencing finite state machine (FSM) 702 table and verifies thecorrectness of individual parameters) of each API call. If an error isdetected, primitive validator 704 terminates the call and returns theerror to the application immediately. Otherwise, primitive validator 704calls conferencing primitive dispatcher 708, which determines whichsubsystem primitives to invoke next.

Conferencing primitive dispatcher 708 dispatches and executes the nextconferencing API primitive to start or continue to carry out the servicerequested by the application. Primitive dispatcher 708 may be invokedeither directly from primitive validator 704 (i.e., to start the firstof a set of conferencing API primitives) or from conferencing callback706 to continue the unfinished processing (for asynchronous API calls).Primitive dispatcher 708 chooses the conferencing API primitives basedon the information of the current state, the type of message/event, andthe next primitive being scheduled by the previous conferencing APIprimitive.

After collecting and analyzing the completion status from eachsubsystem, primitive dispatcher 708 either (1) returns the concludedmessage back to the conferencing application by returning a message orinvoking the application-provided callback routine or (2) continues toinvoke another primitive to continue the unfinished processing.

There are a set of primitives (i.e., comm primitives 712, videoprimitives 716, and audio primitives 720) implemented for each API call.Some primitives are designed to be invoked from a callback routine tocarry out the asynchronous services.

The subsystem callback routine (i.e., comm callback 710) returns thecompletion status of an asynchronous call to the comm subsystem toconferencing callback 706, which will conduct analysis to determine theproper action to take next. The comm callback 710 is implemented as aseparate thread of execution (vthread.exe) that receives the callbackMicrosoft® Windows messages from the comm manager and then calls VCI DLLto handle these messages.

Conferencing callback 706 returns the completion status of anasynchronous call to the application. Conferencing callback 706 checksthe current message/event type, analyzes the type against the currentconferencing API state and the next primitive being scheduled todetermine the actions to take (e.g., invoke another primitive or returnthe message to the application). If the processing is not complete yet,conferencing callback 706 selects another primitive to continue the restof the processing. Otherwise, conferencing callback 706 returns thecompletion status to the application. The conferencing callback 706 isused only for comm related conferencing API functions; all otherconferencing API functions are synchronous.

The major services supported by conferencing API 506 are categorized asfollows:

Call and Channel Services (establish/terminate a conference call andchannels over the call).

Stream Services (capture, play, record, link, and control the multimediaaudio and video streams).

Data Services (access and manipulate data from the multimedia streams).

Interfacing with the Comm Subsystem

Conferencing API 506 supports the following comm services with the commsubsystem:

Call establishment--place a call to start a conference.

Channel establishment--establish four comm channels for incoming video,incoming audio, outgoing video, and outgoing audio. These 4 channels areopened implicitly as part of call establishment, and not throughseparate APIs. The channel APIs are for other channels (e.g., dataconferencing).

Call termination--hang up a call and close all active channels.

Call Establishment

Establishment of a call between the user of conferencing system A ofFIG. 1 and the user of conferencing system B of FIG. 1 is implemented asfollows:

Conferencing APIs A and B call BeginSession to initialize their commsubsystems.

Conferencing API A calls MakeConnection to dial conferencing API B'snumber.

Conferencing API B receives a CONN₋₋ REQUESTED callback.

Conferencing API B sends the call notification to the graphic userinterface (GUI); and if user B accepts the call via the GUI,conferencing API B proceeds with the following steps.

Conferencing API B calls AcceptConnection to accept the incoming callfrom conferencing API A.

Conferencing APIs A and B receives CONN₋₋ ACCEPTED message.

Conferencing APIs A and B call RegisterChanMgr for channel management.

Conferencing API A calls OpenChannel to open the audio channel.

Conferencing API B receives the Chan₋₋ Requested callback and accepts itvia AcceptChannel.

Conferencing API A receives the Chan₋₋ Accepted callback.

The last three steps are repeated for the video channel and the controlchannel.

Conferencing API A then sends the business card information on thecontrol channel, which conferencing API B receives.

Conferencing API B then turns around and repeats the above 6 steps(i.e., opens its outbound channels for audio/video/control and sends itsbusiness card information on its control channel).

Conferencing APIs A and B then notify the conferencing applications witha CFM₋₋ ACCEPT₋₋ NTFY callback.

Channel Establishment

Video and audio channel establishment is implicitly done as part of callestablishment, as described above, and need not be repeated here. Forestablishing other channels such as data conferencing, the conferencingAPI passes through the request to the comm manager, and sends the commmanager's callback to the user's channel manager.

Call Termination

Termination of a call between users A and B is implemented as follows(assuming user A hangs up):

Conferencing API A unlinks local/remote video/audio streams from thenetwork.

Conferencing API A then calls the comm manager's CloseConnection.

The comm manager implicitly closes all channels, and sends Chan₋₋ Closedcallbacks to conferencing API A.

Conferencing API A closes its remote audio/video Streams on receipt ofthe Chan₋₋ Closed callback for its inbound audio/video channels,respectively.

Conferencing API A then receives the CONN₋₋ CLOSE₋₋ RESP from the commmanager after the call is cleaned up completely. Conferencing API Anotifies its application via a CFM₋₋ HANGUP₋₋ NTFY.

In the meantime, the comm manager on B would have received the hangupnotification, and would have closed its end of all the channels, andnotified conferencing API B via Chan₋₋ Closed.

Conferencing API B closes its remote audio/video streams on receipt ofthe Chan₋₋ Closed callback for its inbound audio/video channels,respectively.

Conferencing API B unlinks its local audio/video streams from thenetwork on receipt of the Chan₋₋ Closed callback for its outboundaudio/video channels, respectively.

Conferencing API B then receives a CONN₋₋ CLOSED notification from itscomm manager. Conferencing API B notifies its application via CFM₋₋HANGUP₋₋ NTFY.

Interfacing with the Audio and Video Subsystems

Conferencing API 506 supports the following services with the audio andvideo subsystems:

Capture/monitor/transmit local video streams.

Capture/transmit local audio streams.

Receive/play remote streams.

Control local/remote streams.

Snap an image from local video stream.

Since the video and audio streams are closely synchronized, the audioand video subsystem services are described together.

Capture/Monitor/Transmit Local Streams

The local video and audio streams are captured and monitored as follows:

Call AOpen to open the local audio stream.

Call VOpen to open the local video stream.

Call ACapture to capture the local audio stream from the local hardware.

Call VCapture to capture the local video stream from the local hardware.

Call VMonitor to monitor the local video stream.

The local video and audio streams are begun to be sent out to the remotesite as follows:

Call ALinkOut to connect the local audio stream to an output networkchannel.

Call VLinkOut to connect the local video stream to an output networkchannel.

The monitoring of the local video stream locally is stopped as follows:

Call VMonitor(off) to stop monitoring the local video stream.

Receive/Play Remote Streams

Remote streams are received from the network and played as follows:

Call AOpen to open the local audio stream.

Call VOpen to open the local video stream.

Call ALinkIn to connect the local audio stream to an input networkchannel.

Call VLinkIn to connect the local video stream to an input networkchannel.

Call APlay to play the received remote audio stream.

Call VPlay to play the received remote video stream.

Control Local/Remote Streams

The local video and audio streams are paused as follows:

Call VLinkout(off) to stop sending local video on the network.

Call AMute to stop sending local audio on the network.

The remote video and audio streams are paused as follows:

If CF₋₋ PlayStream(off) is called, conferencing API calls APlay(off) andVPlay(off).

The local/remote video/audio streams are controlled as follows:

Call ACntl to control the gains of a local audio stream or the volume ofthe remote audio stream.

Call VCntl to control such parameters as the brightness, tint, contrast,color of a local or remote video stream.

Snap an Image from Local Video Streams

A snapshot of the local video stream is taken and returned as an imageto the application as follows:

Call VGrabframe to grab the most current image from the local videostream.

Conferencing API 506 supports the following function calls byconferencing applications 502 and 504 to the video, comm, and audiosubsystems:

    __________________________________________________________________________    CF.sub.-- Init                                                                          Reads in the conferencing configuration parameters (e.g.,                     pathname of                                                                   the directory database and directory name in which the                        conferencing                                                                  software is kept) from an initialization file; loads and                      initializes the                                                               software of the comm, video, and audio subsystems by allocating               and                                                                           building internal data structures; allows the application to                  choose                                                                        between the message and the callback routines to return the                   event                                                                         notifications from the remote site.                                 CF.sub.-- MakeCall                                                                      Makes a call to the remote site to establish a connection for                 conferencing. The call is performed asynchronously.                 CF.sub.-- AcceptCall                                                                    Accepts a call initiated from the remote site based on the                    information                                                                   received in the CFM.sub.-- CALL.sub.-- NTFY message.                CF.sub.-- RejectCall                                                                    Rejects incoming call, if appropriate, upon receiving a                       CFM.sub.-- CALL.sub.-- NTFY message.                                CF.sub.-- HangupCall                                                                    Hangs up a call that was previously established; releases all                 resources,                                                                    including all types of streams and data structures, allocated                 during the                                                                    call.                                                               CF.sub.-- GetCallState                                                                  Returns the current state of the specified call.                    CF.sub.-- CapMon                                                                        Starts the capture of analog video signals from the local                     camera and                                                                    displays the video in the local.sub.-- video.sub.-- window                    which is pre-opened by                                                        the application. This function allows the user to preview                     his/her                                                                       appearance before sending the signals out to the remote site.       CF.sub.-- PlayRcvd                                                                      Starts the reception and display of remote video signals in                   the                                                                           remote.sub.-- video.sub.-- window, which is pre-opened by the                 application; starts                                                           the reception and play of remote audio signals through the                    local                                                                         speaker.                                                            CF.sub.-- Destroy                                                                       Destroys the specified stream group that was created by                       CF.sub.-- CapMon                                                              or CF.sub.-- PlayRcvd. As part of the destroy process, all                    operations (e.g.,                                                             sending/playing) being performed on the stream group will be                  stopped                                                                       and all allocated system resources will be freed.                   CF.sub.-- Mute                                                                          Uses AMute to turn on/off the mute function being performed on                the                                                                           audio stream of a specified stream group. This function will                  temporarily stop or restart the related operations, including                 playing and                                                                   sending, being performed on this stream group. This function                  may be                                                                        used to hold temporarily one audio stream and provide more                    bandwidth                                                                     for other streams to use.                                           CF.sub.-- SnapStream                                                                    Takes a snapshot of the video stream of the specified stream                  group and returns a still image (reference) frame to the                      application buffers indicated by the hBuffer handle.                CF.sub.-- Control                                                                       Controls the capture or playback functions of the local or                    remote video                                                                  and audio stream groups.                                            CF.sub.-- SendStream                                                                    Uses ALinkOut to pause/unpause audio.                               CF.sub.-- GetStreamInfo                                                                 Returns the current state and the audio video control block                   (AVCB)                                                                        data structure, preallocated by the application, of the                       specified stream                                                              groups.                                                             CF.sub.-- PlayStream                                                                    Stops/starts the playback of the remote audio/video streams by                calling                                                                       APlay/VPlay.                                                        __________________________________________________________________________

These functions are defined in further detail later in thisspecification in a section entitled "Data Structures, Functions, andMessages."

In addition, conferencing API 506 supports the following messagesreturned to conferencing applications 502 and 504 from the video, comm,and audio subsystems in response to some of the above-listed functions:

    __________________________________________________________________________    CFM.sub.-- CALL.sub.-- NTFY                                                                 Indicates that a call request initiated from the remote                       site has                                                                      been received.                                                  CFM.sub.-- PROGRESS.sub.-- NTFY                                                             Indicates that a call state/progress notification has been                    received                                                                      from the local phone system support.                            CFM.sub.-- ACCEPT.sub.-- NTFY                                                               Indicates that the remote site has accepted the call                          request issued                                                                locally. Also sent to the accepting application when                          CF.sub.-- AcceptCall completes.                                 CFM.sub.-- REJECT.sub.-- NTFY                                                               Indicates that the remote site has rejected or the local                      site has                                                                      failed to make the call.                                        CFM.sub.-- HANGUP.sub.-- NTFY                                                               Indicates that the remote site has hung up the                  __________________________________________________________________________                  call.                                                       

Referring now to FIG. 8, there is shown a representation of theconferencing call finite state machine (FSM) for a conferencing sessionbetween a local conferencing system (i.e., caller) and a remoteconferencing system (i.e., callee), according to a preferred embodimentof the present invention. The possible conferencing call states are asfollows:

    ______________________________________                                        CCST.sub.-- NULL                                                                            Null State - state of uninitialized                                           caller/callee.                                                  CCST.sub.-- IDLE                                                                            Idle State - state of caller/callee ready                                     to make/receive calls.                                          CCST.sub.-- CALLING                                                                         Calling state - state of caller trying                                        to call callee.                                                 CCST.sub.-- CALLED                                                                          Called state - state of callee being                                          called by caller.                                               CCST.sub.-- CONNECTED                                                                       Call state - state of caller and callee                                       during conferencing session.                                    CCST.sub.-- CLOSING                                                                         A hangup or call cleanup is in progress.                        ______________________________________                                    

At the CCST₋₋ CONNECTED state, the local application may begincapturing, monitoring, and/or sending the local audio/video signals tothe remote application. At the same time, the local application may bereceiving and playing the remote audio/video signals.

Referring now to FIG. 9, there is shown a representation of theconferencing stream FSM for each conferencing system participating in aconferencing session, according to a preferred embodiment of the presentinvention. The possible conferencing stream states are as follows:

    ______________________________________                                        CSST.sub.-- INIT                                                                           Initialization state - state of local and                                     remote streams after                                                          CCST.sub.-- CONNECTED state is first                                          reached.                                                         CSST.sub.-- ACTIVE                                                                         Capture state - state of local stream                                         being captured. Receive state - state of                                      remote stream being received.                                    CSST.sub.-- FAILURE                                                                        Fail state - state of local/remote stream                                     after resource failure.                                          ______________________________________                                    

Conferencing stream FSM represents the states of both the local andremote streams of each conferencing system. Note that the local streamfor one conferencing system is the remote stream for the otherconferencing system.

In a typical conferencing session between a caller and a callee, boththe caller and callee begin in the CCST₋₋ NULL call state of FIG. 8. Theconferencing session is initiated by both the caller and callee callingthe function CF₋₋ Init to initialize their own conferencing systems.Initialization involves initializing internal data structures,initializing communication and configuration information, opening alocal directory data base, verifying the local user's identity, andretrieving the user's profile information from the database. The CF₋₋Init function takes both the caller and callee from the CCST₋₋ NULL callstate to the CCST₋₋ IDLE call state. The CF₋₋ Init function also placesboth the local and remote streams of both the caller and callee in theCSST₋₋ INIT stream state of FIG. 9.

Both the caller and callee call the CF₋₋ CapMon function to startcapturing local video and audio signals and playing them locally, takingboth the caller and callee local stream from the CSST₋₋ INIT streamstate to the CSST₋₋ ACTIVE stream state. Both the caller and callee maythen call the CF₋₋ Control function to control the local video and audiosignals, leaving all states unchanged.

The caller then calls the CF₋₋ MakeCall function to initiate a call tothe callee, taking the caller from the CSST₋₋ IDLE call state to theCSST₋₋ CALLING call state. The callee receives and processes a CFM₋₋CALL₋₋ NTFY message indicating that a call has been placed from thecaller, taking the callee from the CSST₋₋ IDLE call state to the CCST₋₋CALLED call state. The callee calls the CF₋₋ AcceptCall function toaccept the call from the caller, taking the callee from the CSST₋₋CALLED call state to the CSST₋₋ CONNECTED call state. The callerreceives and processes a CFM₋₋ ACCEPT₋₋ NTFY message indicating that thecallee accepted the call, taking the caller from the CCST₋₋ CALLING callstate to the CSST₋₋ CONNECTED call state.

Both the caller and callee then call the CF₋₋ PlayRcvd function to beginreception and play of the video and audio streams from the remote site,leaving all states unchanged. Both the caller and callee call the CF₋₋SendStream function to start sending the locally captured video andaudio streams to the remote site, leaving all states unchanged. Ifnecessary, both the caller and callee may then call the CF₋₋ Controlfunction to control the remote video and audio streams, again leavingall states unchanged. The conferencing session then proceeds with nochanges to the call and stream states. During the conferencing session,the application may call CF₋₋ Mute, CF₋₋ PlayStream, or CF₋₋ SendStream.These affect the state of the streams in the audio/video managers, butnot the state of the stream group.

When the conferencing session is to be terminated, the caller calls theCF₋₋ HangupCall function to end the conferencing session, taking thecaller from the CCST₋₋ CONNECTED call state to the CSST₋₋ IDLE callstate. The callee receives and processes a CFM₋₋ HANGUP₋₋ NTFY messagefrom the caller indicating that the caller has hung up, taking thecallee from the CCST₋₋ CONNECTED call state to the CCST₋₋ IDLE callstate.

Both the caller and callee call the CF₋₋ Destroy function to stopplaying the remote video and audio signals, taking both the caller andcallee remote streams from the CSST₋₋ ACTIVE stream state to the CSST₋₋INIT stream state. Both the caller and callee also call the CF₋₋ Destroyfunction to stop capturing the local video and audio signals, takingboth the caller and callee local streams from the CSST₋₋ ACTIVE streamstate to the CSST₋₋ INIT stream state.

This described scenario is just one possible scenario. Those skilled inthe art will understand that other scenarios may be constructed usingthe following additional functions and state transitions:

If the callee does not answer within a specified time period, the callerautomatically calls the CF₋₋ HangupCall function to hang up, taking thecaller from the CSST₋₋ CALLING call state to the CSST₋₋ IDLE call state.

The callee calls the CF₋₋ RejectCall function to reject a call from thecaller, taking the callee from the CSST₋₋ CALLED call state to theCSST₋₋ IDLE call state. The caller then receives and processes a CFM₋₋REJECT₋₋ NTFY message indicating that the callee has rejected thecaller's call, taking the caller from the CSST₋₋ CALLING call state tothe CSST₋₋ IDLE call state.

The callee (rather than the caller) calls the CF₋₋ HangupCall functionto hang up, taking the callee from the CCST₋₋ CONNECTED call state tothe CSST₋₋ IDLE call state. The caller receives a CFM₋₋ HANGUP₋₋ NTFYmessage from the callee indicating that the callee has hung up, takingthe caller from the CSST₋₋ CONNECTED call state to the CSST₋₋ IDLE callstate.

The CF₋₋ GetCallState function may be called by either the caller or thecallee from any call state to determine the current call state withoutchanging the call state.

During a conferencing session, an unrecoverable resource failure mayoccur in the local stream of either the caller or the callee causing thelocal stream to be lost, taking the local stream from the CSST₋₋ ACTIVEstream state to the CSST₋₋ FAILURE stream state. Similarly, anunrecoverable resource failure may occur in the remote stream of eitherthe caller or the callee causing the remote stream to be lost, takingthe remote stream from the CSST₋₋ ACTIVE stream state to the CSST₋₋FAILURE stream state. In either case, the local site calls the CF₋₋Destroy function to recover from the failure, taking the failed streamfrom the CSST₋₋ FAILURE stream state to the CSST₋₋ INIT stream state.

The CF₋₋ GetStreamInfo function may be called by the application fromany stream state of either the local stream or the remote stream todetermine information regarding the specified stream groups. The CF₋₋SnapStream and CF₋₋ RecordStream functions may be called by theapplication for the local stream in the CSST₋₋ ACTIVE stream state orfor the remote stream (CF₋₋ RecordStream only) in the CSST₋₋ ACTIVEstream state. All of the functions described in this paragraph leave thestream state unchanged.

Video Subsystem

The video subsystem of conferencing system 100 of FIG. 5 comprises videoAPI 508, video manager 516, video capture driver 522, and host interface526 running on host processor 202 of FIG. 2 and video microcode 530running on video board 204. The following sections describe each ofthese constituents of the video subsystem.

Video API

Video API 508 of FIG. 5 provides an interface between audio/videoconferencing application 502 and the video subsystem. Video API 508provides the following services:

    __________________________________________________________________________    Capture Service                                                                             Captures a single video stream continuously from a local                      video                                                                         hardware source, for example, a video camera or VCR, and                      directs the                                                                   video stream to a video software output sink (i.e., a                         network                                                                       destination).                                                   Monitor Service                                                                             Monitors the video stream being captured from the local                       video                                                                         hardware in the local video window previously opened by                       the                                                                           application.                                                                  Note: This function intercepts and displays a video stream                    at the                                                                        hardware board when the stream is first captured. This                        operation is                                                                  similar to a "Short circuit" or a UNIX tee and is different                   from the                                                                      "play" function. The play function gets and displays the                      video stream                                                                  at the host. In conferencing system 100, the distinction                      between                                                                       monitor and play services is that one is on the board and                     the other at                                                                  the host. Both are carried out on the host (i.e., software                    playback).                                                                    Rather, the distinction is this: monitor service intercepts                   and displays,                                                                 on the local system, a video stream that has been captured                    with the                                                                      local hardware (generated locally). By contrast, play                         service operates                                                              on a video stream that has been captured on a remote                          system's                                                                      hardware and then sent to the local system (generated                         remotely).                                                      Pause Service Suspends capturing or playing of an active video stream;                      resumes                                                                       capturing or playing of a previously suspended video                          stream.                                                         Image Capture Grabs the most current complete still image (called a                         reference frame)                                                              from the specified video stream and returns it to the                         application in the                                                            Microsoft ® DIB (Device-Independent Bitmap) format.         Play Service  Plays a video stream continuously by consuming the video                      frames from                                                                   a video software source (i.e., a network source).               Link-In Service                                                                             Links a video network source to be the input of a video                       stream played                                                                 locally. This service allows applications to change                           dynamically the                                                               software input source of a video stream.                        Link-Out Service                                                                            Links a network source to be the output of a video stream                     captured                                                                      locally. This service allows applications to change                           dynamically the                                                               software output source of a video stream.                       Control Service                                                                             Controls the video stream "on the fly," including adjusting                   brightness,                                                                   contrast, frame rate, and data rate.                            Information Service                                                                         Returns status and information about a specified video                        stream.                                                         Initialization/Configuration                                                                Initializes the video subsystem and calculates the cost, in                   terms                                                                         of system resources, required to sustain certain video                        configurations. These costs can be used by other subsystems                   to                                                                            determine the optimum product configuration for the given                     system.                                                         __________________________________________________________________________

Video API 508 supports the following function calls by audio/videoconferencing application 502 to the video subsystem:

    __________________________________________________________________________    VOpen  Opens a video stream with specified attributes by allocating all              necessary system resources (e.g., internal data structures) for               it.                                                                    VCapture                                                                             Starts/stops capturing a video stream from a local video hardware             source, such as a video camera or VCR.                                 VMonitor                                                                             Starts/stops monitoring a video stream captured from local a                  video                                                                         camera or VCR.                                                         VPlay  Starts/stops playing a video stream from a network, or remote,                video                                                                         source. When starting to play, the video frames are consumed from             a                                                                             network video source and displayed in a window pre-opened by the              application.                                                           VLinkIn                                                                              Links/unlinks a network . . . to/from a specified video stream,               which will                                                                    be played/is being played locally.                                     VLinkOut                                                                             Links/unlinks a network . . . to/from a specified video stream,               which will                                                                    be captured/is being captured from the local camera or VCR.            VGrabframe                                                                           Grabs the most current still image (reference frame) from a                   specified                                                                     video stream and returns the frame in an application-provided                 buffer.                                                                VPause Starts/stops pausing a video stream captured/played locally.           VCntl  Controls a video stream by adjusting its parameters (e.g.,                    tint/contrast,                                                                frame/data rate).                                                      VGetInfo                                                                             Returns the status (VINFO and state) of a video stream.                VClose Closes a video stream and releases all system resources allocated             for                                                                           this stream.                                                           VInit  Initializes the video subsystem, starts capture and playback                  applications, and                                                             calculates system utilization for video configurations.                VShutdown                                                                            Shuts down the video subsystem and stops the capture and playback             applications.                                                          VCost  Calculates and reports the percentage CPU utilization required to             support a                                                                     given video stream.                                                    __________________________________________________________________________

These functions are defined in further detail later in thisspecification in a section entitled "Data Structures, Functions, andMessages."

Referring now to FIG. 10, there is shown a representation of the videoFSM for the local video stream and the remote video stream of aconferencing system during a conferencing session, according to apreferred embodiment of the present invention. The possible video statesare as follows:

    __________________________________________________________________________    VST.sub.-- INIT                                                                        Initial state - state of local and remote video streams after                 the                                                                           application calls the CF.sub.-- Init function.                       VST.sub.-- OPEN                                                                        Open state - state of the local/remote video stream after                     system                                                                        resources have been allocated.                                       VST.sub.-- CAPTURE                                                                     Capture state - state of local video stream being captured.          VST.sub.-- LINKOUT                                                                     Link-out state - state of local video stream being linked to                  video output                                                                  (e.g., network output channel or output file).                       VST.sub.-- LINKIN                                                                      Link-in state - state of remote video stream being linked to                  video input                                                                   (e.g., network input channel or input file).                         VST.sub.-- PLAY                                                                        Play state - state of remote video stream being played.              VST.sub.-- ERROR                                                                       Error state - state of local/remote video stream after a system               resource                                                                      failure occurs.                                                      __________________________________________________________________________

In a typical conferencing session between a caller and a callee, boththe local and remote video streams begin in the VST₋₋ INIT video stateof FIG. 10. The application calls the VOpen function to open the localvideo stream, taking the local video stream from the VST₋₋ INIT videostate to the VST₋₋ OPEN video state. The application then calls theVCapture function to begin capturing the local video stream, taking thelocal video stream from the VST₋₋ OPEN video state to the VST₋₋ CAPTUREvideo state. The application then calls the VLinkOut function to linkthe local video stream to the video output channel, taking the localvideo stream from the VST₋₋ CAPTURE video state to the VST₋₋ LINKOUTvideo state.

The application calls the VOpen function to open the remote videostream, taking the remote video stream from the VST₋₋ INIT video stateto the VST₋₋ OPEN video state. The application then calls the VLinkInfunction to link the remote video stream to the video input channel,taking the remote video stream from the VST₋₋ OPEN video state to theVST₋₋ LINKIN video state. The application then calls the VPlay functionto begin playing the remote video stream, taking the remote video streamfrom the VST₋₋ LINKIN video state to the VST₋₋ PLAY video state. Theconferencing session proceeds without changing the video states ofeither the local or remote video stream.

When the conferencing session is to be terminated, the application callsthe VClose function to close the remote video channel, taking the remotevideo stream from the VST₋₋ PLAY video state to the VST₋₋ INIT videostate. The application also calls the VClose function to close the localvideo channel, taking the local video stream from the VST₋₋ LINKOUTvideo state to the VST₋₋ INIT video state.

This described scenario is just one possible video scenario. Thoseskilled in the art will understand that other scenarios may beconstructed using the following additional functions and statetransitions:

The application calls the VLinkOut function to unlink the local videostream from the video output channel, taking the local video stream fromthe VST₋₋ LINKOUT video state to the VST₋₋ CAPTURE video state.

The application calls the VCapture function to stop capturing the localvideo stream, taking the local video stream from the VST₋₋ CAPTURE videostate to the VST₋₋ OPEN video state.

The application calls the VClose function to close the local videostream, taking the local video stream from the VST₋₋ OPEN video state tothe VST₋₋ INIT video state.

The application calls the VClose function to close the local videostream, taking the local video stream from the VST₋₋ CAPTURE video stateto the VST₋₋ INIT video state.

The application calls the VClose function to recover from a systemresource failure, taking the local video stream from the VST₋₋ ERRORvideo state to the VST₋₋ INIT video state.

The application calls the VPlay function to stop playing the remotevideo stream, taking the remote video stream from the VST₋₋ PLAY videostate to the VST₋₋ LINKIN video state.

The application calls the VLinkIn function to unlink the remote videostream from the video input channel, taking the remote video stream fromthe VST₋₋ LINKIN video state to the VST₋₋ OPEN video state.

The application calls the VClose function to close the remote videostream, taking the remote video stream from the VST₋₋ OPEN video stateto the VST₋₋ INIT video state.

The application calls the VClose function to close the remote videostream, taking the remote video stream from the VST₋₋ LINKIN video stateto the VST₋₋ INIT video state.

The application calls the VClose function to recover from a systemresource failure, taking the remote video stream from the VST₋₋ ERRORvideo state to the VST₋₋ INIT video state.

The VGetInfo and VCntl functions may be called by the application fromany video state of either the local or remote video stream, except forthe VST₋₋ INIT state. The VPause and VGrabFrame functions may be calledby the application for the local video stream from either the VST₋₋CAPTURE or VST₋₋ LINKOUT video states or for the remote video streamfrom the VST₋₋ PLAY video state. The VMonitor function may be called bythe application for the local video stream from either the VST₋₋ CAPTUREor VST₋₋ LINKOUT video states. All of the functions described in thisparagraph leave the video state unchanged.

Video Manager

Referring now to FIG. 11, there is shown a block diagram of the softwarecomponents of video manager (VM) 516 of FIG. 5, according to a preferredembodiment of the present invention. Video manager 516 is implementedusing five major components:

    __________________________________________________________________________    Library  (VM DLL 1102) A Microsoft ® Windows Dynamic Link Library                  (DLL) that provides the library of functions of video API 508.       Capture  (VCapt EXE 1104) A Microsoft ® Windows application                        (independently                                                                executable control thread with stack, message queue, and data)                which                                                                         controls the capture and distribution of video frames from video              board                                                                         204.                                                                 Playback (VPlay EXE 1106) A Microsoft ® Windows application which                  controls                                                                      the playback (i.e., decode and display) of video frames received              from                                                                          either the network or a co-resident capture application.             Network Library                                                                        (Netw DLL 1108) A Microsoft ® Windows DLL which provides                  interfaces to send and receive video frames across a network or               in a                                                                          local loopback path to a co-resident playback application. The                Netw                                                                          DLL hides details of the underlying network support from the                  capture                                                                       and playback applications and implements (in a manner hidden                  from                                                                          those applications) the local loopback function.                     Audio-Video                                                                            (AVSync DLL 1110) A Microsoft ® Windows DLL                      Synchronization                                                                        which provides interfaces to enable the                              Library  synchronization of video frames with a separate stream of audio               frames                                                                        for the purposes of achieving "lip-synchronization." AVSync DLL               1110 supports the implementation of an audio-video                            synchronization                                                               technique described later in this specification.                     __________________________________________________________________________

The five major components, and their interactions, define how the VMimplementation is decomposed for the purposes of an implementation. Inaddition, five techniques provide full realization of theimplementation:

    __________________________________________________________________________    Stream Restart                                                                          A technique for initially starting, and restarting, a video                   stream. If a                                                                  video stream consists entirely of encoded "delta" frames, then                the                                                                           method of stream start/restart quickly supplies the decoder                   with a "key"                                                                  or reference frame. Stream restart is used when a video stream                becomes out-of-sync with respect to the audio.                      Synchronization                                                                         An audio-video synchronization technique for synchronizing a                  sequence, or stream, of video frames with an external audio                   source.                                                             Bit Rate Throttling                                                                     A technique by which the video stream bit rate is controlled so               that                                                                          video frame data coexists with other video conferencing                       components.                                                                   This technique is dynamic in nature and acts to "throttle" the                video                                                                         stream (up and down) in response to higher priority requests                  (higher                                                                       than video data priority) made at the network interface.            Multiple Video                                                                          A technique by which multiple video formats                         Formats   are used to optimize transfer, decode, and display costs when                 video                                                                         frames are moved between video board 204 and host processor                   202.                                                                          This technique balances video frame data transfer overhead with               host                                                                          processor decode and display overhead in order to implement                   efficiently                                                                   a local video monitor.                                              Self-Calibration                                                                        A self-calibration technique which is used to determine the                   amount of                                                                     motion video PC system can support. This allows conferencing                  system                                                                        100 to vary video decode and display configurations in order to               run on                                                                        a range of PC systems. It is particularly applicable in                       software-                                                                     playback systems.                                                   __________________________________________________________________________

Capture/Playback Video Effects

This subsection describes an important feature of the VM implementationthat has an impact on the implementation of both the capture andplayback applications (VCapt EXE 1104 and VPlay EXE 1106). One of thekey goals of VM capture and playback is that while local Microsoft®Windows application activity may impact local video playback, it neednot effect remote video playback. That is, due to the non-preemptivenature of the Microsoft® Windows environment, the VPlay application maynot get control to run, and as such, local monitor and remote playbackwill be halted. However, if captured flames are delivered as a part ofcapture hardware interrupt handling, and network interfaces areaccessible at interrupt time, then captured video flames can betransmitted on the network, regardless of local conditions.

With respect to conferencing system 100, both of these conditions aresatisfied. This is an important feature in an end-to-end conferencingsituation, where the local endpoint is unaware of remote endpointprocessing, and can only explain local playback starvation as a resultof local activity. The preferred capture and playback application designensures that remote video is not lost due to remote endpoint activity.

Video Stream Restart

The preferred video compression method for conferencing system 100(i.e., ISDN rate video or IRV) contains no key flames (i.e., referenceflames). Every frame is a delta (i.e., difference) frame based on thepreceding decoded video frame. In order to establish a complete videoimage, IRV dedicates a small part (preferably 1/85th) of each deltaframe to key frame data. The part of an IRV delta frame that is key iscomplete and does not require inter-frame decode. The position of thekey information is relative, and is said to "walk" with respect to adelta frame sequence, so that the use of partial key information may bereferred to as the "walking key frame."

Referring now to FIG. 12, there is shown a representation of a sequenceof N walking key flames. For a walking key frame of size 1/N, the kthframe in a sequence of N frames, where (k<=N), has its kth componentconsisting of key information. On decode, that kth component is completeand accurate. Provided frame k+1 is decoded correctly, the kth componentof the video stream will remain accurate, since it is based on a kth keycomponent and a k+1 correct decode. A complete key frame is generatedevery N frames in order to provide the decoder with up-to-date referenceinformation within N frames.

For a continuous and uninterrupted stream of video frames, the walkingkey frame provides key information without bit-rate fluctuations thatwould occur if a complete key frame were sent at regular intervals.However, without a complete key frame, video startup requires collectingall walking key frame components, which requires a delay of N frames. Ifvideo startup/restart occurs often, this can be problematic, especiallyif N is large. For example, at 10 frames per second (fps) with N=85, thestartup/restart time to build video from scratch is 8.5 seconds.

In order to accelerate IRV stream startup and restart, an IRV capturedriver "Request Key Frame" interface is used to generate a complete keyframe on demand. The complete key frame "compresses" N frames of walkingkey frames into a single frame, and allows immediate stream startup onceit is received and decoded. Compressed IRV key frames for (160×120)video images are approximately 6-8 KBytes in length. Assuming an ISDNbandwidth of 90 kbits dedicated to video, ISDN key frame transmissiontakes approximately 0.5-0.6 seconds to transmit. Given a walking keyframe size of 1/85(N=85), and a frame rate of 10 fps, use of a completekey frame to start/restart a video stream can decrease the startup delayfrom 8.5 secs to approximately 1/2 sec.

In order for walking key frame compression to be successful, the deltaframe rate must be lowered during key frame transmission. Delta framesgenerated during key frame transmission are likely to be "out-of-sync"with respect to establishing audio-video synchronization, and given thesize of a key frame, too many delta frames will exceed the overall ISDNbandwidth. The IRV capture driver bit rate controller takes into accountkey frame data in its frame generation logic and decreases frame rateimmediately following a key frame.

A key frame once received may be "out-of-sync" with respect to the audiostream due to its lengthy transmission time. Thus, key frames will bedecoded but not displayed, and the video stream will be "in-sync" onlywhen the first follow-on delta frame is received. In addition, the"way-out-of-sync" window is preferably sized appropriately so that keyframe transmission does not cause the stream to require repeatedrestarts.

Once it is determined that a stream requires restart, either as part ofcall establishment or due to synchronization problems, the localendpoint requiring the restart transmits a restart control message tothe remote capture endpoint requesting a key frame. The remote capturesite responds by requesting its capture driver to generate a key frame.The key frame is sent to the local endpoint when generated. The endpointrequesting the restart sets a timer immediately following the restartrequest. If a key frame is not received after an adequate delay, therestart request is repeated.

Audio/Video Synchronization

Video manager 516 is responsible for synchronizing the video stream withthe audio stream in order to achieve "lip-synchronization." Because ofthe overall conferencing architecture, the audio and video subsystems donot share a common clock. In addition, again because of system design,the audio stream is a more reliable, lower latency stream than the videostream. For these reasons, the video stream is synchronized by relyingon information regarding capture and playback audio timing.

For VM audio/video (A/V) synchronization, audio stream packets aretime-stamped from an external clock at the time they are captured. Whenan audio packet is played, its timestamp represents the current audioplayback time. Every video frame captured is stamped with a timestamp,derived from the audio system, that is the capture timestamp of the lastaudio packet captured. At the time of video playback (decode anddisplay, typically at the remote endpoint of a video conference), thevideo frame timestamp is compared with the current audio playback time,as derived from the audio system.

Two windows, or time periods, δ₁ and δ₂, are defined, with δ₁ <δ₂, aspart of VM initialization. Let V_(T) be the timestamp for a given videoframe, and let A_(T) be the current audio playback time when the videoframe is to be played. A/V synchronization is defined as follows:

1. If |A_(T) -V_(T) |≦δ₁, then the video stream is "in-sync" and playednormally (i.e., decoded and displayed immediately).

2. If δ₁ <|A_(T) -V_(T) |≦δ₂, then the video stream is "out-of-sync" anda "hurry-up" technique is used to attempt re-synchronization. If a videostream remains out-of-sync for too many consecutive frames, then itbecomes "way-out-of-sync" and requires a restart.

3. If δ₂ <|A_(T) -V_(T) |, then the video stream is "way-out-of-sync"and requires a restart.

Because of the overall design of conferencing system 100, a video streamsent from one endpoint to another is "behind" its corresponding audiostream. That is, the transmission and reception of a video frame takeslonger than the transmission and reception of an audio frame. This isdue to the design of video and audio capture and playback sites relativeto the network interface, as well as video and audio frame sizedifferences. In order to compensate for this, the audio system allowscapture and playback latencies to be set for an audio stream. Audiocapture and playback latencies artificially delay the capture andplayback of an audio stream.

As part of the VLinkOut function, video manager 516 calls audio manager520 to set an audio capture latency. As part of the VLinkIn function,video manager 516 calls audio manager 520 to set an audio playbacklatency. Once the latencies are set, they are preferably not changed.The capture and playback latency values are specified in milliseconds,and defined as part of VM initialization. They may be adjusted as partof the Calibration process.

In order to attempt re-synchronization when a stream is not too far"out-of-sync" as defined by the above rules, an feature called"Hurry-up" is used. When passing a video frame to the codec for decode,if hurry-up is specified, then the codec performs frame decode to a YUVintermediate format but does not execute the YUV-to-RGB colorconversion. Though the output is not color converted for RGB graphicsdisplay, the hurry-up maintains the playback decode stream for followingframes. When Hurry-up is used, the frame is not displayed. By decreasingthe decode/display cost per frame and processing frames on demand (thenumber of frames processed for playback per second can vary), it ispossible for a video stream that is out-of-sync to become in-sync.

Bit Rate Throttling

Conferencing system 100 supports a number of different media: audio,video, and data. These media are prioritized in order to share thelimited network (e.g., ISDN) bandwidth. A priority order of(highest-to-lowest) audio, data, and video is designated. In thisscheme, network bandwidth that is used for video will need to give wayto data, when data conferencing is active (audio is not compromised). Inorder to implement the priority design, a mechanism for dynamicallythrottling the video bit stream is used. It is a self-throttling system,in that it does not require input from a centralized bit ratecontroller. It both throttles down and throttles up a video bit streamas a function of available network bandwidth.

A latency is a period of time needed to complete the transfer of a givenamount of data at a given bit rate. For example, for 10 kbits at 10kbits/sec, latency=1. A throttle down latency is the latency at which abit stream is throttled down (i.e., its rate is lowered), and a throttleup latency is the latency at which a bit stream is throttled up (i.e.,its rate is increased).

Multiple Video Formats

Conferencing system 100 presents both a local monitor display and aremote playback display to the user. A digital video resolution of(160×120) is preferably used as capture resolution for ISDN-based videoconferencing (i.e., the resolution of a coded compressed video stream toa remote site). (160×120) and (320×24) are preferably used as the localmonitor display resolution. (320×240) resolution may also be used forhigh-resolution still images. Generating the local monitor display bydecompressing and color converting the compressed video stream would becomputationally expensive. The video capture driver 522 of FIG. 5simultaneously generates both a compressed video stream and anuncompressed video stream. Video manager 516 makes use of theuncompressed video stream to generate the local monitor display. Videomanager 516 may select the format of the uncompressed video stream to beeither YUV-9 or 8-bits/pixel (bpp) RGB--Device Independent Bitmap (DIB)format. For a (160×120) local monitor, the uncompressed DIB video streammay be displayed directly. For a (320×240) monitor, a (160×120) YUV-9format is used and the display driver "doubles" the image size to(320×240) as part of the color conversion process.

In the RGB and YUV-9 capture modes, RGB or YUV data are appended tocapture driver IRV buffers, so that the capture application (VCapt EXE1104) has access to both fully encoded IRV frames and either RGB or YUVdata. Conferencing system 100 has custom capture driver interfaces toselect either RGB capture mode, YUV capture mode, or neither.

Self-Calibration

CPU, I/O bus, and display adapter characteristics vary widely fromcomputer to computer. The goal of VM self-calibration is to supportsoftware-based video playback on a variety of PC platforms, withouthaving to "hard-code" fixed system parameters based on knowledge of thehost PC. VM self-calibration measures a PC computer system in order todetermine the decode and display overheads that it can support. VMself-calibration also offers a cost function that upper-layer softwaremay use to determine if selected display options, for a given videocompression format, are supported.

There are three major elements to the self-calibration:

1. The calibration of software decode using actual video decompresscycles to measure decompression costs. Both RGB/YUV capture mode and IRVframes are decoded in order to provide accurate measurement of local(monitor) and remote video decode. YUV (160×120) and YUV (320×240)formats are also decoded (color converted) to provide costs associatedwith the YUV preview feature of the video subsystem.

2. A calibration of PC displays, at varying resolutions, using actualvideo display cycles to measure display costs.

3. A video cost function, available to applications, that takes as inputframe rate, display rate, display resolution, video format, andmiscellaneous video stream characteristics, and outputs a systemutilization percentage representing the total system cost for supportinga video decompress and display having the specified characteristics.

The calibration software detects a CPU upgrade or display drivermodification in order to determine if calibration is to be run, prior toan initial run on a newly installed system.

VM DLL

Referring again to FIG. 11, video manager dynamic link library (VM DLL)WB is a video stream "object manager." That is, with few exceptions, allVM DLL interfaces take a "Video Stream Object Handle" (HVSTRM) as input,and the interfaces define a set of operations or functions on a streamobject. Multiple stream objects may be created.

Video API 508 defines all of external interfaces to VM DLL WB. There arealso a number of VM internal interfaces to VM DLL WB that are used byVCapt EXE WC, VPlay EXE WD, Netw DLL WE, and AVSync DLL WF for thepurposes of manipulating a video stream at a lower level than thatavailable to applications. The vm.h file, provided to applications thatuse VM DLL WF, contains a definition of all EPS and VM internalinterfaces. EPS interfaces are prefixed with a `V`; VM internalinterfaces are prefixed with a `VM`. Finally, there are a number of VMprivate interfaces, available only to the VM DLL code, used to implementthe object functions. For example, there are stream object validationroutines. The self-calibration code is a separate module linked with theVM DLL code proper.

Video API calls, following HVSTRM and parameter validation, aretypically passed down to either VCapt or VPlay for processing. This isimplemented using the Microsoft® Windows SDK SendMessage interface.SendMessage takes as input the window handle of the target applicationand synchronously calls the main window proc of that application. Aspart of VM initialization, VM starts execution of the applications,VCapt and VPlay. As part of their WinMain processing, theseapplicationss make use of a VMRegister interface to return their windowhandle to VM DLL WB. From registered window handles, VM DLL WB is ableto make use of the SendMessage interface. For every video API interface,there is a corresponding parameter block structure used to passparameters to VCapt or VPlay. These structures are defined in the vm.hfile. In addition to the WinExec startup and video API interface calls,VM DLL WB can also send a shutdown message to VCapt and VPlay fortermination processing.

Immediately following the successful initialization of VCapt and VPlay,VM 516 calls the interface `videoMeasure` in order to runself-calibration. The VCost interface is available, at run-time, toreturn measurement information, per video stream, to applications.

VCapt EXE

The video capture application (VCapt EXE WC) implements all details ofvideo frame capture and distribution to the network, including:

Control of the ISVR capture driver.

Video format handling to support IRV and RGB/YUV capture mode.

Video frame capture callback processing of captured video frames.

Copy followed by PostMessage transfer of video frames to local playbackapplication (VPlay EXE).

Transmission, via Netw DLL WE, of video flames to the network.

Mirror, zoom, camera video attributes, and miscellaneous capture streamcontrol processing.

Restart requests from a remote endpoint.

Shutdown processing.

VCapt EXE WC processing may be summarized as a function of theMicrosoft® Windows messages as follows:

WINMAIN

Initialize application.

Get VCapt EXE initialization (INI) settings.

Open ISVR driver.

Register window handle (and status) with VM DLL WB.

Enter Microsoft® Windows message loop.

WM₋₋ VCAPTURE₋₋ CALL (ON)

Register audio callback with audio manager 520.

Set audio capture latency with audio manager 520.

Initialize the ISVR capture stream based on stream object attributes.

WM₋₋ VLINKOUT₋₋ CALL (ON)

Register Netw callback handler for transmission completion handling.

Initialize bit rate throttling parameters.

WM₋₋ MONITOR₋₋ DATA₋₋ RTN

Decrement reference count on video frame (user context buffers).

WM₋₋ PLAY₋₋ DATA₋₋ RTN

Add buffer back to capture driver.

This message is only in loopback case of remote playback--preferably fortesting only.

WM₋₋ RESTART₋₋ STREAM

Request key frame from capture driver.

WM₋₋ VCNTL₋₋ CALL

Adjust video stream controls based on VCntl parameters (from VM DLL WB).

WM₋₋ PLAYBACK

Get stream format type (IRV, YUV).

Set ISVR RGB/YUV capture mode controls: If IRV (160×120) playback thenRGB; if IRV 320×240 playback, then YUV.

This message is from local playback application (VPlay EXE WD) inresponse to local window (monitor) size changes.

WM₋₋ SHUTDOWN

Disable capture; includes closing the capture driver.

Un-initializes capture application.

DestroyWindow.

VCapt Capture Callback is a key component of the VCapt EXE application.VCapt Capture Callback processes individual frames received, ininterrupt context, from the capture driver (ISVR.DRV). The main steps ofcallback processing are:

Time stamp the video frame using AVSync DLL WF.

Set the packet sequence number of the frame (for network errordetection).

If the video stream is in the Monitor state, then copy the frame out ofinterrupt context into a local monitor playback frame first-infirst--out (FIFO) device. If the video format is YUV, then only theframe header is copied, since YUV data does not go to the network, andis not "real-time."

If the video stream is in the LinkOut state of FIG. 10, then call theNETWSendFrame function to send the frame to the remote playback site,and then add the frame buffer back to the capture driver. Also, useinterface DataRateThrottleDown to adjust the video bit rate, as needed.

VPlay EXE

The video playback application (VPlay EXE WD) implements all details ofvideo playback, including:

Opening an instance of the IRV playback codec for each playback stream:local monitor and remote playback.

Maintaining display mode attributes for each stream, based on playbackwindow sizes

Maintain palette "awareness" for each video stream.

Receive video frames for decompress and display.

Filter video frames using AVSync DLL WF and playback frame FIFO state.

Restart video stream as necessary.

Decompress video frames via Microsoft® Windows 3.1 SendDriverMessageCodec interface.

Display video frames via Microsoft® GDI or DrawDIB interfaces.

Handle VM DLL messages generated as a result of video API interfacecalls.

Handle application shutdown.

In order to encapsulate decode and display attributes for a video streamin a "Display Object," references to a Display Object are passed tointernal VPlay procedures. The structure of the Display Object isdefined in the vplay.h include file.

VPlay EXE WD processing may be summarized as a function of theMicrosoft® Windows messages as follows:

WINMAIN

Initialize application.

Get VPlay initialization (INI) settings.

Register window handle (and status) with VM DLL WB.

Enter Microsoft® Windows message loop.

WM₋₋ TIMER

Kill the outstanding restart timer.

If the stream associated with the message is still in the restart state,then RestartStream.

Initialize the ISVR capture stream based on stream object attributes.

WM₋₋ MONITOR₋₋ DATA

Validate stream state (MONITOR) and video frame data.

ProcessPlayFrame.

Set reference count to 0 (copy frame FIFO).

WM₋₋ PLAY₋₋ DATA

Validate stream state (PLAY) and video frame data.

ProcessPlayFrame.

NETWPostFrame to return frame buffer to the network.

WM₋₋ VMONITOR₋₋ CALL (ON)

Get video stream attributes and determine internal stream playbackvalues.

Set up codec for stream; set up decompress structures.

RestartStream.

WM₋₋ VPLAY₋₋ CALL (ON)

Get video stream attributes and determine internal stream playbackvalues.

Set up codec for stream; set up decompress structures.

RestartStream.

WM₋₋ VLINKIN₋₋ CALL (ON)

AVRegisterMonitor to set AVSync audio manager callback.

AVSetLatency to set audio manager playback latency.

NETWRegisterIn to register receive data complete callbacks from networkand post video frame network buffers.

WM₋₋ VCNTL₋₋ CALL

Adjust video stream controls (via codec) based on VCntl parameters (fromVM DLL WB).

WM₋₋ VGRABFRAME₋₋ CALL

Copy out the current RGB display buffer for the stream.

WM₋₋ MEASURE₋₋ BEGIN

Turn on video statistics gathering.

WM₋₋ MEASURE₋₋ END

Return decode and display playback statistics for the stream.

WM₋₋ MEASURE₋₋ BEGIN

Turn on video statistics gathering.

WM₋₋ SHUTDOWN

Clean up codec.

DestroyWindow.

Unregister Class.

The `ProcessPlayFrame` procedure is a key component of the playbackapplication (VPlay EXE WD). It processes individual frames received, inuser context, from either the VCapt capture callback, in the case oflocal monitor playback, or from the Netw receive data complete callback,in the case of remote playback. The main steps of `ProcessPlayFrame`processing are:

Send the video frame through the `SyncFilter`.

If the frame is "way-out-of-sync," then restart the stream.

If the frame is "out-of-sync," then `hurry₋₋ up`=TRUE.

Else, `hurry₋₋ up`=FALSE.

Based on the stream display frequency attribute, determine if the frameshould be displayed. If the frame is not to be displayed, then `hurry₋₋up`=TRUE; else `hurry₋₋ up`=FALSE.

If the stream is REMOTE, then decode with IRV decompress.

If the stream is LOCAL, then:

If the stream is IRV (i.e., not RGB/YUV capture mode), then decode withIRV decompress;

Else if the stream is RGB capture mode, then copy to RGB display buffer;

Else if the stream is YUV capture mode, then decode with IRV ColorConvert;

Else if the stream is YUV, then decode with IRV Color Convert;

If all frames have been decompressed (no more frames in playback frameFIFO) and `hurry₋₋ up`==FALSE, then Display Frame.

SyncFilter, a procedure used by ProcessPlayFrame, is implemented asfollows:

If the playback frame Fifo length is>AVFrameHighWaterMark, then return("way-out-of-sync").

If the stream is REMOTE, then if there is a Frame Packet Sequence NumberError, then return ("way-out-of-sync").

If the stream is REMOTE, then return (AVFrameSync (StreamObject,FramePtr)).

The first test is important: It states that the number of frames queuedfor playback has exceeded a high water mark, which indicates that VPlayEXE WD has been starved and the stream playback is "way-out-of-sync."The AVFrameSync interface (AVSync DLL WF) is preferably only used withremote streams, since local streams do not have the concept of anassociated audio playback time.

DisplayFrame, a procedure used by ProcessPlayFrame, is implemented asfollows: Based on the stream Display Object mode, use Microsoft® WindowsDrawDib, BitBlt, or Stretchblt to display the frame. The display mode isa function of playback window size and video format resolution.

RestartStream is a procedure that handles details of stream restart. Itsimplementation is:

Clear the playback frame FIFO (the ClearFrameFifo procedure recyclesqueued video frames to the network or VCapt, as needed).

Set the stream state to `RESTART`.

If the stream is LOCAL, then:

If YUV/RGB capture mode is not enabled, then PostMessage (WM₋₋ STREAM₋₋RESTART, 0, 0) to VCapt EXE WC indicating a key frame request. IfYUV/RGB capture mode is enabled, then every captured frame contains aRGB or YUV capture mode key frame, and a key frame request isunnecessary.

Else (stream is REMOTE) NETWSendCntl (WM₋₋ RESTART₋₋ STREAM) to have thenetwork send a restart control message; Set the Key Frame Request timer.

One of the more important areas of the VPlay implementation is its"Palette Awareness" logic. In order that video displays retain propercolors in a palettized environment, VPlay must respond to a Microsoft®Windows palette change and get new palette messages. To accomplish this,VPlay "hooks" the window specified in the WM₋₋ VPLAY₋₋ CALL messageparameter block, so that palette messages to the "hooked" window will betransmitted to a procedure within VPlay that properly handles thepalette management.

Netw DLL

Network library (Netw DLL WE) provides a library of network interfacesdesigned to hide the capture and playback applications from details ofthe underlying network service, including:

Management of network buffers.

Asynchronous interrupt-time callbacks when data is received ortransmission is complete.

Video frame and control message transmission.

Compaction of video frame headers, from Microsoft® Video for Windows(VfW) defined headers to packed headers suitable for low-bandwidthnetworks (e.g., ISDN).

Transparent local loopback of video frames (supports single machinetesting of video subsystem).

Netw DLL WE defines a `SUPERVIDEOHDR` structure, which is an extensionof the `VIDEOHDR` structure defined by Microsoft® Video for Windows. TheVIDEOHDR structure is used by VfW capture and playback applications on asingle PC. The SUPERVIDEOHDR contains the VIDEOHDR structure, plusVM-specific control information, an area where VIDEOHDR data can becompacted for network transmission, and a contiguous frame data buffer.The contiguity of the SUPERVIDEOHDR structure allows the VfW structureto be used without modification by VCapt and VPlay (which are also VfWapplications), while at the same time allowing a video frame to betransmitted on the network in a single operation.

The interfaces provided by the Netw DLL are as follows:

NETWCallbackIn--Callback used for VLinkIn streams; processes receiveddata from the network.

NETWCallbackOut--Callback used for VLinkOut streams; processes sendcompletions from the network.

NETWInit--Initializes network buffers.

NETWRegisterIn--Register a network input channel and post buffers forreceiving data.

NETWRegisterOut--Register a network output channel.

NETWSendCntl--Send a control message.

NETWSendFrame--Send a video frame.

NETWPostFrame--Post a video frame buffer to the network interface.

NETWCleanup--Un-initialize NETW support; buffers, etc.

AVSync DLL

AVSync DLL WF provides a library of interfaces designed to support thecapture and playback applications in the implementation of theaudio-video synchronization technique, including:

Implementing audio system callbacks used to deliver timestamp values.

Implementing audio system latency settings.

Maintaining capture stream and playback stream timestamps.

Video frame comparison with video stream timestamp values.

The interfaces provided by the AVSync DLL are as follows:

AVInit--Initialization. Includes getting critical AV sync values fromINI file.

AVRegisterMonitor--Register timestamp callback for a video stream.

AVUnRegisterMonitor--Unregister timestamp callback for a video stream.

AVSetALatency--Set a capture or playback audio latency value.

AVReSetALatency--Reset a capture or playback audio latency value.

AVFifoHighWaterMark--Return a configuration-defined value for the highwater mark of a video frame FIFO. (Used in VPlay SyncFilter.)

AVFrameTimeStamp--Time stamp a video frame with an associated audiocapture time stamp.

AVFrameSync--Determine if a video frame is "in-sync" as defined for"in-sync," "out-of-sync," and "way-out-of-sync" disclosed earlier inthis specification.

Video Capture Driver

Video capture driver 522 of FIG. 5 follows driver specifications setforth in the Microsoft® Video for Windows (VfW) Developer Kitdocumentation. This documentation specifies a series of applicationprogram interfaces (APIs) to which the video capture driver responds.Microsoft® Video for Windows is a Microsoft extension to the Microsoft®Windows operating system. VfW provides a common framework to integrateaudio and video into an application program. Video capture driver 522extends the basic Microsoft® API definitions by providing six "custom"APIs that provide direct control of enhancements to the standard VfWspecification to enable and control bit rate throttling and local videomonitoring.

Bit rate throttling controls the bit rate of a transmitted videoconference data stream. Bit rate throttling is based on two independentparameters: the quality of the captured video image and the imagecapture frame rate. A user of conferencing system 100 is able to varythe relative importance of these two parameters with a custom capturedriver API. A high-quality image has more fine detail information than alow-quality image.

The data bandwidth capacity of the video conference communicationchannel is fixed. The amount of captured video data to be transmitted isvariable, depending upon the amount of motion that is present in thevideo image. The capture driver is able to control the amount of datathat is captured by changing the quality of the next captured videoframe and by not capturing the next video frame ("dropping" the frame).

The image quality is determined on a frame-by-frame basis using thefollowing equation: ##EQU1## Quality is the relative image quality ofthe next captured frame. A lower Quality number represents a lower imagequality (less image detail). TargetSize is the desired size of acaptured and compressed frame. TargetSize is based on a fixed, desiredcapture frame rate.

Normally, the capture driver captures new video frames at a fixed,periodic rate which is set by the audio/video conference applicationprogram. The capture driver keeps a running total of the availablecommunication channel bandwidth. When the capture driver is ready tocapture the next video frame, it first checks the available channelbandwidth and if there is insufficient bandwidth (due to a large,previously captured frame), then the capture driver delays capturing thenext video frame until sufficient bandwidth is available. Finally, thesize of the captured video frame is subtracted from the availablechannel bandwidth total.

A user of conferencing system 100 may control the relationship betweenreduced image quality and dropped frames by setting the minimum imagequality value. The minimum image quality value controls the range ofpermitted image qualities, from a wide range down to a narrow range ofonly the best image qualities.

Bit rate throttling is implemented inside of the video capture driverand is controlled by the following VfW extension APIs:

    ______________________________________                                        CUSTOM.sub.-- SET.sub.-- DATA.sub.-- RATE                                                          Sets the data rate of the                                                     communications channel.                                  CUSTOM.sub.-- SET.sub.-- QUAL.sub.-- PERCENT                                                       Sets the minimum image                                                        quality value.                                           CUSTOM.sub.-- SET.sub.-- FPS                                                                       Sets the desired capture                                                      frame rate.                                              ______________________________________                                    

The local video monitoring extension to VfW gives the video capturedriver the ability to output simultaneously both a compressed and anon-compressed image data stream to the application, while remainingfully compatible with the Microsoft® VfW interface specification.Without local video monitoring, the audio/video conferencing applicationprogram would be required to decompress and display the image streamgenerated by the capture driver, which places an additional burden onthe host processor and decreases the frame update rate of the displayedimage.

The VfW interface specification requires that compressed image data beplaced in an output buffer. When local video monitoring is active, anuncompressed copy of the same image frame is appended to the outputbuffer immediately following the compressed image data. The capturedriver generates control information associated with the output buffer.This control information reflects only the compressed image block of theoutput buffer and does not indicate the presence of the uncompressedimage block, making local video monitoring fully compatible with otherVfW applications. A "reserved," 32-bit data word in the VfW controlinformation block indicates to a local video monitor aware applicationthat there is a valid uncompressed video image block in the outputbuffer. The application program may then read and directly display theuncompressed video image block from the output buffer.

The uncompressed image data may be in either Device Independent Bitmap(DIB) or YUV9 format. DIB format images may be displayed directly on thecomputer monitor. YUV9 format images may be increased in size whileretaining image quality. YUV9 images are converted into DIB formatbefore they are displayed on the computer monitor.

The capture driver allows the uncompressed video image to be capturedeither normally or mirrored (reversed left to right). In normal mode,the local video monitoring image appears as it is viewed by a videocamera--printing appears correctly in the displayed image. In mirroredmode, the local video monitoring image appears as if it were beingviewed in a mirror.

The CUSTOM₋₋ SET₋₋ DIB₋₋ CONTROL extension API controls the local videomonitoring capabilities of the video capture driver.

Custom APIs for Video Capture Driver

The CUSTOM₋₋ SET₋₋ FPS message sets the frame rate for a video capture.This message can only be used while in streaming capture mode.

The CUSTOM₋₋ SET₋₋ KEY message informs the driver to produce one keyframe as soon as possible. The capture driver will commonly produce onedelta frame before the key. Once the key frame has been encoded, deltaflames will follow normally.

The CUSTOM₋₋ SET₋₋ DATA₋₋ RATE message informs the driver to set anoutput data rate. This data rate value is in KBits per second andtypically corresponds to the data rate of the communications channelover which the compressed video data will be transmitted.

The CUSTOM₋₋ SET₋₋ QUAL₋₋ PERCENT message controls the relationshipbetween reducing the image quality and dropping video frames when thecompressed video data stream size exceeds the data rate set by theCUSTOM₋₋ SET₋₋ DATA₋₋ RATE message. For example, a CUSTOM₋₋ SET₋₋ QUAL₋₋PERCENT value of 0 means that the driver should reduce the image qualityas much as possible before dropping frames and a value of 100 means thatvideo frames should be dropped before the image quality is lowered.

The CUSTOM₋₋ SET₋₋ DIB₋₋ CONTROL message controls the 8-bit DIB/YUV9format image output when the IRV compression format has been selected.The IRV driver is able to simultaneously generate the IRV compresseddata stream plus an uncompressed image in either DIB or YUV9 format. Ifenabled, the IRV driver can return the DIB image in either (80×60) or(160×120) pixel resolution. The (160×120) image is also available inYUV9 format. All images are available in either mirrored (reversed leftto right) or a normal image. This API controls the following fourparameters:

DIB enable/disable

Mirrored/normal image

The DIB image size

Image data format

The default condition is for the uncompressed image to be disabled. Onceset, these control flags remains in effect until changed by anotherCUSTOM₋₋ SET₋₋ DIB₋₋ CONTROL message. The uncompressed image data isappended to the video data buffer immediately following the compressedIRV image data. The uncompressed DIB or YUV9 data have the bottomscan-line data first and the top scan-line data last in the buffer.

The CUSTOM₋₋ SET₋₋ VIDEO message controls the video demodulatorCONTRAST, BRIGHTNESS, HUE (TINT), and SATURATION parameters. These videoparameters are also set by the capture driver at initialization and viathe Video Control dialog box.

Video Microcode

The video microcode 530 of FIG. 5 running on video board 204 of FIG. 2performs video compression. The preferred video compression technique isdisclosed in later sections of this specification starting with thesection entitled "Compressed Video Bitstream."

Audio Subsystem

The audio subsystem provides full duplex audio between two conferencingsystems 100. The audio streams in both directions preferably runvirtually error free, and do not break up due to activity on hostprocessor 202. While the video subsystem is responsible forsynchronizing video with audio, the audio subsystem provides aninterface to retrieve synchronization information and for control overaudio latency. The synchronization information and latency control isprovided through an interface internal to the audio and videosubsystems.

The audio subsystem provides an interface for control of the audiostreams. Output volume, selection of an audio compression method, samplesize, and sample rate are examples of audio attributes that may beselected or adjusted through the interface. In addition to controllingaudio attributes, the audio subsystem provides an interface to sendaudio streams out to the network, receive and play audio streams fromthe network, and monitor the local audio stream.

When audio/comm board 206 is not being used for video conferencing, theMicrosoft® Wave interface provides access to the stereo audio codec(SAC). Wave driver 524 supports all of the predefined Microsoft® samplerates, full duplex audio, both eight and sixteen bit samples, and monoor stereo audio. Wave driver 524 provides the audio subsystem with aprivate interface that allows the Wave driver to be disabled.

In a preferred embodiment, the Microsoft® Wave interface performs recordand playback of audio during a conferencing session. To achieve this,the audio subsystem and the Wave implementation cooperate during videoconferencing so that the audio stream(s) can be split between the Waveinterface and the source/sink of the audio subsystem.

Referring now to FIG. 13, there is shown a block diagram of thearchitecture of the audio subsystem, according to a preferred embodimentof the present invention. The audio subsystem is structured as a "DSPapplication." Conforming with the DSP architecture forces the audiosubsystem's implementation to be split between host processor 202 andaudio/comm board 206. Conceptually, audio tasks on the audio/comm boardcommunicate directly with a counterpart on the host processor. Forexample, Wave driver 524 (on the host processor) communicates directlywith Wave task 534 (on the audio/comm board). In FIG. 13, thesecommunications are represented by broken lines representing virtualconnections.

The bulk of the audio subsystem is implemented on the audio/comm boardas a Spectron SPOX DSP operating system task. The portion of the audiosubsystem on the host processor provides an interface to control theSPOX® audio task. The programming interface to the audio subsystem isimplemented as a DLL on top of DSP interface 528. The DLL will translateall function calls into DSP messages and respond to messages passed fromaudio task 538 to the host processor.

The audio task 538 (running on the audio/comm board) responds to controlinformation and requests for status from audio manager 520 (running onthe host processor). The audio task is also responsible for hardwaremonitoring of the audio input source on the audio output sink. Amajority of the audio task's execution time is spent fulfilling itsthird and primary responsibility: full duplex audio communicationbetween two conferencing systems.

The conferencing application's interface to the audio subsystem isimplemented on the host processor, and the audio processing and controlis implemented on the audio/comm board as a SPOX® operating system task.These two software components interface with each other through messagespassed through the DSP interface 528 of FIG. 5.

Referring again to FIG. 1, in order for the audio subsystem to achievefull duplex communication between two conferencing systems, there is anetwork connection (i.e., ISDN line 110) between two conferencingsystems. Both conferencing systems run the same software. This allowsthe audio task on one conferencing system to communicate with anotherinstantiation of itself on the other conferencing system. The ISDNconnection is full duplex. There are two B-Channels in each direction.Logical audio channels flowing through the ISDN connection are providedby the network tasks and have no physical representation. The audio taskon each of the conferencing systems is responsible for playing back thecompressed audio generated on the remote system, and for transferringthe compressed audio generated locally to the remote system.

Referring now to FIGS. 1 and 13, audio samples generated on conferencingsystem A are first sampled by microphone 104, digitized by the stereoaudio codec (SAC), filtered and compressed by the stack of devicedrivers 1304, and delivered to the audio task 538. The audio taskpacketizes the compressed audio (by time stamping the audioinformation), and then sends the audio to comm task 540 for delivery tothe remote system. The audio samples consumed (i.e., played back) byconferencing system A are delivered by the comm task after conferencingsystem B has gone through the same process as conferencing system A togenerate and send a packet. Once conferencing system A has the audiopacket generated by conferencing system B, the comm task records thetime stamp, and sends the packet down the device stack 1302 to bedecompressed and sent to the codec (i.e., audio hardware 1306). As theremote audio samples are being transferred to the codec, the codec maymix them with local audio samples (depending on whether the local systemis in the monitor state or not), and finally sends the samples to theattached speaker 108.

Audio API

Referring again to FIG. 5, the audio API 512 for the audio subsystem isan internal programming interface used by other software components ofthe conferencing system, specifically video manager 516 and theconferencing API 506. The audio API is a library that is linked in withthe calling application. The audio API translates the proceduralinterface into DriverProc messages. See Microsoft® Device DriverDevelopment Kit (DDK) and Software Development Kit (SDK) for thedefinitions of the DriverProc entry point and installable devicedrivers. The audio API layer also keeps the state machine for the audiosubsystem. This allows the state machine to be implemented only once forevery implementation of the audio subsystem.

Audio API 512 of FIG. 5 provides an interface between audio/videoconferencing application 502 and the audio subsystem. Audio API 512provides the following services:

    __________________________________________________________________________    Capture Service                                                                             Captures a single audio stream continuously from a local                      audio                                                                         hardware source, for example, a microphone, and directs the                   audio                                                                         stream to a audio software output sink (i.e., a network                       destination).                                                   Monitor Service                                                                             Monitors the audio stream being captured from the local                       audio                                                                         hardware by playing the audio stream locally.                                 Note: This function intercepts and displays a audio stream                    at the                                                                        hardware board when the stream is first captured. This                        operation is                                                                  similar to a "Short circuit" or a UNIX tee and is different                   from the                                                                      "play" function. The play function gets and displays the                      audio stream                                                                  at the host.                                                    Play Service  Plays an audio stream continuously by consuming the audio                     data from                                                                     an audio software source (i.e., a network source).              Link-In Service                                                                             Links an audio network source to be the input of an audio                     stream                                                                        played locally. This service allows applications to change                    dynamically                                                                   the software input source of an audio stream.                   Link-Out Service                                                                            Links a network source to be the output of an audio stream                    captured                                                                      locally. This service allows applications to change                           dynamically the                                                               software output source of an audio stream.                      Control Service                                                                             Controls the audio stream "on the fly," including adjusting                   gain,                                                                         volume, and latency.                                            Information Service                                                                         Returns requested information regarding the specified video                   stream.                                                         Initialization/Configuration                                                                Initialize at OPEN time.                                        __________________________________________________________________________

Audio API 512 supports the following function calls by audio/videoconferencing application 502 to the audio subsystem:

    __________________________________________________________________________    AGetNumDevs                                                                            Retrieves the number of different audio managers installed on                 the                                                                           system.                                                              AGetDevCaps                                                                            Fills the ADevCaps structure with information regarding the                   specified                                                                     audio manager.                                                       AOpen    Opens an audio stream with specified attributes by allocating                 all                                                                           necessary system resources (e.g., internal data structures) for               it.                                                                  ACapture Starts/stops capturing an audio stream from a local audio                     hardware                                                                      source, such as a microphone.                                        AMonitor Starts/stops monitoring an audio stream captured from a local                 microphone.                                                          APlay    Starts/stops playing an audio stream by consuming the audio data              from                                                                          an audio network source.                                             ALinkIn  Links/unlinks a network input channel or an input file to/from                the                                                                           specified audio stream that will be played or is being played                 locally.                                                             ALinkOut Links/unlinks a network output channel to/from the specified                  audio                                                                         stream that will be captured or is being captured from the                    local                                                                         microphone.                                                          ACntl    Controls an audio stream by adjusting its parameters (e.g.,                   gain,                                                                         volume).                                                             AGetInfo Returns the status (AINFO and state) of an audio stream.             AClose   Closes an audio stream and releases all system resources                      allocated for                                                                 this stream.                                                         ARegisterMonitor                                                                       Registers an audio stream monitor.                                   APacketNumber                                                                          Returns the packet number of the current audio packet being                   played                                                                        back or recorded.                                                    __________________________________________________________________________

These functions are defined in further detail later in thisspecification in a section entitled "Data Structures, Functions, andMessages."

Referring now to FIG. 14, there is shown a representation of the audioFSM for the local audio stream and the remote audio stream of aconferencing system during a conferencing session, according to apreferred embodiment of the present invention. The possible audio statesare as follows:

    __________________________________________________________________________    AST.sub.-- INIT                                                                        Initial state - state of local and remote audio streams after                 the                                                                           application calls the CF.sub.-- Init function.                       AST.sub.-- OPEN                                                                        Open state - state of the local/remote audio stream after                     system                                                                        resources have been allocated.                                       AST.sub.-- CAPTURE                                                                     Capture state - state of local audio stream being captured.          AST.sub.-- LINKOUT                                                                     Link-out state - state of local audio stream being                            linked/unlinked to                                                            audio output (e.g., network output channel or output file).          AST.sub.-- LINKIN                                                                      Link-in state - state of remote audio stream being                            linked/unlinked to                                                            audio input (e.g., network input channel or input file).             AST.sub.-- PLAY                                                                        Play state - state of remote audio stream being played.              AST.sub.-- ERROR                                                                       Error state - state of local/remote audio stream after a system               resource                                                                      failure occurs.                                                      __________________________________________________________________________

In a typical conferencing session between a caller and a callee, boththe local and remote audio streams begin in the AST₋₋ INIT audio stateof FIG. 14. The application calls the AOpen function to open the localaudio stream, taking the local audio stream from the AST₋₋ INIT audiostate to the AST₋₋ OPEN audio state. The application then calls theACapture function to begin capturing the local audio stream, taking thelocal audio stream from the AST₋₋ OPEN audio state to the AST₋₋ CAPTUREaudio state. The application then calls the ALinkOut function to linkthe local audio stream to the audio output channel, taking the localaudio stream from the AST₋₋ CAPTURE audio state to the AST₋₋ LINKOUTaudio state.

The application calls the AOpen function to open the remote audiostream, taking the remote audio stream from the AST₋₋ INIT audio stateto the AST₋₋ OPEN audio state. The application then calls the ALinkInfunction to link the remote audio stream to the audio input channel,taking the remote audio stream from the AST₋₋ OPEN audio state to theAST₋₋ LINKIN audio state. The application then calls the APlay functionto begin playing the remote audio stream, taking the remote audio streamfrom the AST₋₋ LINKIN audio state to the AST₋₋ PLAY audio state. Theconferencing session proceeds without changing the audio states ofeither the local or remote audio stream.

When the conferencing session is to be terminated, the application callsthe AClose function to close the remote audio channel, taking the remoteaudio stream from the AST₋₋ PLAY audio state to the AST₋₋ INIT audiostate. The application also calls the AClose function to close the localaudio channel, taking the local audio stream from the AST₋₋ LINKOUTaudio state to the AST₋₋ INIT audio state.

This described scenario is just one possible audio scenario. Thoseskilled in the art will understand that other scenarios may beconstructed using the following additional functions and statetransitions:

The application calls the ALinkOut function to unlink the local audiostream from the audio output channel, taking the local audio stream fromthe AST₋₋ LINKOUT audio state to the AST₋₋ CAPTURE audio state.

The application calls the ACapture function to stop capturing the localaudio stream, taking the local audio stream from the AST₋₋ CAPTURE audiostate to the AST₋₋ OPEN audio state.

The application calls the AClose function to close the local audiostream, taking the local audio stream from the AST₋₋ OPEN audio state tothe AST₋₋ INIT audio state.

The application calls the AClose function to close the local audiostream, taking the local audio stream from the AST₋₋ CAPTURE audio stateto the AST₋₋ INIT audio state.

The application calls the AClose function to recover from a systemresource failure, taking the local audio stream from the AST₋₋ ERRORaudio state to the AST₋₋ INIT audio state.

The application calls the APlay function to stop playing the remoteaudio stream, taking the remote audio stream from the AST₋₋ PLAY audiostate to the AST₋₋ LINKIN audio state.

The application calls the ALinkIn function to unlink the remote audiostream from the audio input channel, taking the remote audio stream fromthe AST₋₋ LINKIN audio state to the AST₋₋ OPEN audio state.

The application calls the AClose function to close the remote audiostream, taking the remote audio stream from the AST₋₋ OPEN audio stateto the AST₋₋ INIT audio state.

The application calls the AClose function to close the remote audiostream, taking the remote audio stream from the AST₋₋ LINKIN audio stateto the AST₋₋ INIT audio state.

The application calls the AClose function to recover from a systemresource failure, taking the remote audio stream from the AST₋₋ ERRORaudio state to the AST₋₋ INIT audio state.

The AGetDevCaps and AGetNumDevs functions may be called by theapplication from any audio state of either the local or remote audiostream. The AGetInfo, ACntl, and APacketNumber functions may be calledby the application from any audio state of either the local or remoteaudio stream, except for the AST₋₋ INIT state. The AMonitor function maybe called by the application for the local audio stream from either theAST₋₋ CAPTURE or AST₋₋ LINKOUT audio states. The ARegisterMonitorfunction may be called by the application for the local audio streamfrom the AST₋₋ LINKOUT audio state or for the remote audio stream fromeither the AST₋₋ LINKIN or AST₋₋ PLAY audio states. All of the functionsdescribed in this paragraph leave the audio state unchanged.

Audio Manager

The function of audio manager 520 of FIGS. 5 and 13, a Microsoft®Windows installable device driver, is to interface with the audio task538 running on the audio/comm board 206 through the DSP interface 532.By using the installable device driver model, many differentimplementations of the audio manager may co-exist on the same machine.Audio manager 520 has two logical parts:

A device driver interface (DDI) that comprises the messages the devicedriver expects, and

An interface with DSP interface 528.

Audio Manager Device Driver Interface

The device driver interface specifies the entry points and messages thatthe audio manager's installable device driver supports. The entry pointsare the same for all installable device drivers (i.e., Microsoft® WEP,LIBENTRY, and DriverProc). All messages are passed through theDriverProc entry point. Messages concerning loading, unloading,initializing, opening, closing, and configuring the device driver arepredefined by Microsoft®. Those messages specific to the audio managerare defined in relation to the constant MSG₋₋ AUDIO₋₋ MANAGER (thesemessage will range from DRV₋₋ RESERVED to DRV₋₋ USER as defined inMicrosoft® WINDOWS.H). All messages that apply to an audio stream areserialized (i.e., the application does not have more than one messageper audio stream pending).

The installable device driver implementing the audio manager responds tothe open protocol messages defined by Microsoft®. The expected messages(generated by a Microsoft® OpenDriver SDK call to installable devicedrivers) and the drivers response are as follows:

    __________________________________________________________________________    DRV.sub.-- LOAD                                                                        Reads any configuration parameters associated with the driver.                Allocates any memory required for execution. This call is only                made                                                                          the first time the driver is opened.                                 DRV.sub.-- ENABLE                                                                      Set up the Wave driver to work with the audio manager. Ensures                that                                                                          an audio/comm board is installed and functional. For audio/comm               board 206 of FIG. 2, this means the DSP interface 532 is                      accessible.                                                                   This call is only made the first time the driver is opened.          DRV.sub.-- OPEN                                                                        Allocates the per application data. This includes information                 such as                                                                       the callback and the application instance data. If this is an                 input or                                                                      output call, starts the DSP audio task and sets up                            communication                                                                 between host processor and the DSP audio task (e.g., sets up                  mail                                                                          boxes, registers callbacks). The audio manager may be opened                  once                                                                          for input, once for output (i.e., it supports one full duplex                 conversation), and any number of times for device capabilities                query.                                                                        This call is made each time OpenDriver is called.                    __________________________________________________________________________

These three messages are generated in response to a single applicationcall (OpenDriver).

The OpenDriver call is passed a pointer to the following structure inthe lParam2 of the parameter of the call:

    ______________________________________                                        typedef struct OpenAudioMangerStruct {                                        BOOL             GetDevCaps;                                                  LPACAPS          IpACaps;                                                     DWORD            SynchronousError;                                            LPAINFO          AInfo;                                                       DWORD            dwCallback;                                                  DWORD            dwCallbackInstance;                                          DWORD            dwFlags;                                                     DWORD            wField;                                                      } OpenAudioManager, FAR * lpOpenAudioManager;                                 ______________________________________                                    

All three messages receive this parameter in their lParam2 parameter. Ifthe open is being made for either capture or playback, the caller isnotified in response to an asynchronous event (i.e., DSP₋₋ OPENgenerated by dspOpenTask). If the open is being done in order to querythe devices capabilities (indicated by the field OpenAudioManager withGetDevCaps being set to TRUE), the open is synchronous and only fails ifthe board cannot be accessed.

The DRV₋₋ OPEN handler always checks for error conditions, beginsexecution of the audio thread, and allocates per audio stream stateinformation. Once the open command sets state indicating that a DRV₋₋OPEN is pending, it will initiate execution of the audio thread via theDSP interface.

dspOpenTask posts a callback when the audio thread has successfullybegun. This callback is ignored unless it indicates an error. The taskwill call back to the audio driver once it has allocated all thenecessary resources on the board. The callback from the DSP interfacesets the internal state of the device driver to indicate that the threadis running. Once the task has responded, a DRV₋₋ OPEN message call back(i.e., post message) back to the caller of the open command with thefollowing values:

Param1 equals A₋₋ OK, and

Param2 contains the error message returned by the board.

The installable device driver will respond to the close protocolmessages defined by Microsoft®. The expected messages (generated by theMicrosoft® SDK CloseDriver call to installable device drivers) and thedrivers response are as follows:

    ______________________________________                                        DRV.sub.-- CLOSE                                                                          Frees the per application data allocated in                                   DRV.sub.-- OPEN message.                                          DRV.sub.-- DISABLE                                                                        Shuts down the DSP audio task. Enables the                                    Wave driver and Wave task. Frees all                                          memory allocated during DRV.sub.-- LOAD.                          DRV.sub.-- FREE                                                                           Ignored.                                                          ______________________________________                                    

This call sequence is symmetric with respect to the call sequencegenerated by OpenDriver.

It has the same characteristics and behavior as the open sequence does.Namely, it receives one to three messages from the CloseDriver calldependent on the driver's state and it generates one callback perCloseDriver call. Three messages are received when the driver's finalinstance is being closed. Only the DRV₋₋ CLOSE message is generated forother CloseDriver calls.

DRV₋₋ CLOSE message closes the audio thread that corresponds to theaudio stream indicated by HASTRM. The response to the close message isin response to a message sent back from the board indicating that thedriver has closed. Therefore, this call is asynchronous. There is a racecondition on close. The audio task could close down after the close fromthe DRV₋₋ has completed. If this is the case, the DRIVER could beunloaded before the callback occurs. If this happens, the callback willcall into nonexistent code. The full driver close sequence is preferablygenerated on the last close as indicated by the SDK. See Microsoft®Programmers Reference, Volume 1: Overview, pages 445-446).

The installable device driver implementing the host portion of the audiosubsystem recognizes specific messages from the audio API layer.Messages are passed to the driver through the SendDriverMessage and arereceived by DrvProc. The messages and their expected parameters are:

    ______________________________________                                        Message        1Param1        1Param2                                         ______________________________________                                        AM.sub.-- CAPTURE                                                                            BOOL           LPDWORD                                         AM.sub.-- MUTE BOOL           LPDWORD                                         AM.sub.-- PLAY BOOL           LPDWORD                                         AM.sub.-- LINKIN                                                                             FAR * ALinkStruct                                                                            LPDWORD                                         AM.sub.-- LINKOUT                                                                            FAR * ALinkStruct                                                                            LPDWORD                                         AM.sub.-- CTRL FAR * ControlStruct                                                                          LPDWORD                                         AM.sub.-- REGISTERMON                                                                        LPRegisterInfo LPDWORD                                         AM.sub.-- PACKETNUMBER                                                                       NULL           NULL                                            ______________________________________                                    

AM₋₋ CAPTURE Message

The AM₋₋ CAPTURE message is sent to the driver whenever the audiomanager function ACapture is called. This message uses Param1 to pass aboolean value and Param2 is used for a long pointer to a DWORD wheresynchronous errors can be returned. The stream handle will be checked toensure that it is a capture stream, and that there is not a messagepending. The state is not checked because the interface module shouldkeep the state. If an error state is detected, the appropriate errormessage will be returned. The BOOL passed in Param2 indicates whether tostart or stop capturing. A value of TRUE indicates capturing shouldstart, a value of FALSE that capturing should be stopped. ACAPTURE₋₋TMSG is sent to the audio task running on the audio/comm board and themessage pending flag is set for that stream. When the audio taskreceives the message via the DSP interface, it will change its state andcall back to the driver. When the driver receives this callback, it willcall back/post message to the appropriate entity on the host processor,and cancel the message pending flag. This call is a toggle, no state iskept by the driver, and it will call the DSP interface regardless of thevalue of the BOOL.

AM₋₋ MUTE Message

The AM₋₋ MUTE message is sent to the driver whenever the audio managerfunction AMute is called. This message uses Param1 to pass a booleanvalue and Param2 a long pointer to a DWORD for a synchronous errorvalue. The stream handle is checked to ensure that it is a capturestream, and that no messages are pending. If an error state is detected,the appropriate error message is returned. The BOOL passed in Param1indicates whether to staff or stop muting. A value of TRUE indicatesmuting should start, a value of FALSE that muting should be turned off.The driver posts the message AMUTE₋₋ TMSG to the audio task through theDSP interface, and sets the message pending flag. When the driverreceives this callback, it will call back/post message to theappropriate entity on the host processor, and then cancel the messagepending flag.

AM₋₋ PLAY Message

The AM₋₋ PLAY message is sent to the driver whenever the audio managerfunction APlay is called. This message uses Param1 to pass an audiomanager stream handle (HASTRM) and Param2 to pass a boolean value. TheAPlay message handler checks the stream handle to ensure that it is aplayback stream, and verifies that there is not a message pendingagainst this stream. If an error is detected, a call back/post messageis made immediately. The BOOL passed in Param1 indicates whether tostart or stop playing the remote stream. A value of TRUE indicates thatplayback should start, a value of FALSE that playback should stop. TheAPLAY₋₋ TMSG is posted to the audio task through the DSP interface andthe message pending flag is set for this stream. When the callback isprocessed, the caller is notified (via callback/post message), andfinally the message pending flag for this stream is canceled.

AM₋₋ LINKIN Message

The AM₋₋ LINKIN message is sent to the driver whenever the audio managerfunction ALinkIn is called. Param1 passes the Audio Manager streamhandle (HASTRM). lParam2 contains a pointer to the following structure:

    ______________________________________                                        typedef struct.sub.-- ALinkStruct {                                                     BOOL    ToLink;                                                               CHANID  ChanId;                                                     } ALinkStruct, FAR * lpALinkStruct;                                           ______________________________________                                    

ToLink contains a BOOL value that indicates whether the stream is beinglinked in or unlinked (TRUE is linked in and FALSE is unlinked). If noerror is detected and ToLink is TRUE, the channel and the playbackstream should be linked together. This is done by sending the Audio Taskthe ALINKIN₋₋ TMSG message with the channel ID as a parameter. Thiscauses the Audio Task to link up with the specified comm channel andbegin playing incoming audio. Channel ID is sent as a parameter toALINKIN₋₋ TMSG implying that the channel ID is valid in the boardenvironment as well as the host processor. In response to this message,the audio manager registers with the comm task as the owner of thestream.

Breaking the link between the audio stream handle and the channel ID isdone when the ToLink field is set to FALSE. The audio manager sends theALINKIN₋₋ TMSG to the task along with the channel ID. Since the link ismade, the audio task responds to this message by unlinking the specifiedchannel ID (i.e., it does not play any more audio).

Errors that the host task will detect are as follows:

The channel ID does not represents a valid read stream.

The audio stream handle is already linked or unlinked (detected on hostprocessor).

The audio stream handle is not a playback handle.

If those or any interface errors (e.g., message pending) are detected,the callback associated with this stream is notified immediately. If noerrors are detected, the ALINKIN₋₋ TMSGS is issued to the DSP interfaceand the message pending flag is set for this stream. Upon receiving thecallback for this message, the callback associated with this stream ismade, and finally the message pending flag is unset.

AM₋₋ LINKOUT Message

The AM₋₋ LINKOUT message is sent to the driver whenever the audiomanager function ALinkOut is called. Param1 passes the audio managerstream handle (HASTRM). lParam2 contains a pointer to the followingstructure:

    ______________________________________                                        typedef struct.sub.-- ALinkStruct {                                                     BOOL    ToLink;                                                               CHANID  ChanId;                                                     } ALinkStruct, FAR * lpALinkStruct;                                           ______________________________________                                    

ToLink contains a BOOL value that indicates whether the stream is beinglinked out or unlinked (TRUE is linked out and FALSE is unlinked). If noerror is detected and ToLink is TRUE, the channel and the audio instream should be linked together. This is done by sending the Audio Taskthe ALINKOUT₋₋ TMSG message with the channel ID as a parameter. TheAudio Task responds to this by sending audio over the logical channelthrough the comm task. Channel ID is sent as a parameter to ALINKOUT₋₋TMSG implying that the channel ID is valid in the board environment aswell as on the host processor.

Breaking the link between the audio stream handle and the channel ID isdone when ToLink field is set to FALSE. The audio manager sends theALINKOUT₋₋ TMSG to the task along with the channel ID. Since the link ismade, the Audio Task responds to this message by unlinking the specifiedchannel ID (i.e., it does not send any more audio).

Errors that the host task detects are as follows:

The channel ID does not represents a valid write stream.

The audio stream handle is already linked or unlinked (detected on thehost processor).

The audio stream handle is not an audio handle.

If those or any interface errors (e.g., message pending) are detected,the callback associated with this stream is notified immediately. If noerrors are detected, the ALINKOUT₋₋ TMSG is issued to the DSP interfaceand the message pending flag is set for this stream. Upon receiving thecallback for this message, the callback associated with this stream ismade, and finally the message pending flag is unset.

AM₋₋ CRTL Message

The AM₋₋ CRTL message is sent to the driver whenever the audio managerfunction ACtrl is called. Param1 contains the HASTRM (the audio streamhandle) and Param2 contains a long pointer to the following structure:

    ______________________________________                                        typedef struct.sub.-- ControlStruct {                                                  LPAINFO  lpAinfo;                                                             DWORD    flags;                                                      } ControlStruct, FAR * lpControlStruct;                                       ______________________________________                                    

The flags field is used to indicate which fields of the AINFO structurepointed to by lpAinfo are to be considered. The audio manager tracks thestate of the audio task and only adjust it if the flags and AINFOstructure actually indicate change.

Error checking will be for:

Valid audio stream state.

Values and fields adjusted are legal.

Pending calls on the current stream.

If there are any errors to be reported, the audio manager immediatelyissues a callback to the registered callback indicating the error.

If there are no errors, the audio manager makes the audio stream stateas pending, saves a copy of the structure and the adjustment to be made,and begins making the adjustments one by one. The adjustments are madeby sending the audio task the ACNTL₋₋ TMSG message with three argumentsin the dwArgs array. The arguments identify the audio stream, the audioattribute to change, and the new value of the audio attribute. Each timethe audio task processes one of these messages, it generates a callbackto the audio manager. In the callback, the audio manager updates thestream's attributes, removes that flag from the flags field of thestructure (remember this is an internal copy), and sends another ACNTL₋₋TMSG for the next flag. Upon receiving the callback for the last flag,the audio manager calls back the registered callback for this stream,and unsets the pending flag for this stream.

AM₋₋ REGISTERMON Message

The AM₋₋ REGISTERMON message is sent to the driver whenever the audiomanager function ARegisterMonitor is called. Param2 contains a LPDWORDfor synchronous error messages and Param1 contains a long pointer to thefollowing structure:

    ______________________________________                                        typedef struct.sub.-- RegisterMonitor                                                DWORD    dwCallback;                                                          DWORD    dwCallbackInstance;                                                  DWORD    dwflags;                                                             DWORD    dwRequestFrequency;                                                  LPDWORD  lpdwSetFrequency                                              } RegisterMonitor, FAR * LPRegisterMonitor;                                   ______________________________________                                    

The audio manager calls this routine back with information about thestatus of the audio packet being recorded/played back by the audio task.There may only be one callback associated with a stream at a time. Ifthere is already a monitor associated with the stream when this call ismade, it is replaced.

Errors detected by the audio manager are:

Call pending against this audio stream.

Bad stream handle.

These errors are reported to the callback via the functions returnvalues (i.e., they are reported synchronously).

If the registration is successful, the audio manager sends the audiotask a AREGISTERMON₋₋ TMSG via the DSP Interface. The first DWORD ofdwArgs array contains the audio stream ID, and the second specifies thecallback frequency. In response to the AREGISTERMON₋₋ TMSG, the audiotask calls back with the current audio packet number. The audio taskthen generates a callback for every N packets of audio to the audiomanager. The audio manager callback generates a callback to the monitorfunction with AM₋₋ PACKET₋₋ NUMBER as the message, A₋₋ OK as PARAM1, andthe packet number as PARAM2. When the audio stream being monitored isclosed, the audio manager calls back the monitor with A₋₋ STREAM₋₋CLOSED as PARAM1.

AM₋₋ PACKETNUMBER Message

The AM₋₋ PACKETNUMBER message is sent to the driver whenever the audiomanager function APacketNumber is called. Param1 and Param2 are NULL. Ifa monitor is registered for this stream handle, the audio task is sent aAPACKETNUMBER₋₋ TMSG message. In response to this message, the audiotask calls back the audio manager with the current packet number. Theaudio manager in turn calls back the registered monitor with the currentpacket number.

This is one of the few calls/messages that generates both synchronousand asynchronous error messages. The messages have been keptasynchronous whenever possible to be consistent with the programmingmodel. Synchronous errors that are detected are:

The stream has no monitor registered.

Bad HASTRM handle.

If there is no monitor registered (i.e., no callback function to call)or if the HASTRM handle is invalid (again no callback to call), theerror is given synchronously (i.e., as a return value to the function).Asynchronous errors are as follows:

There is a call pending on this audio stream.

The stream is in an invalid state (i.e., not AST₋₋ LINKOUT or AST₋₋PLAY).

The asynchronous errors are given to the monitor function, not thecallback registered with the audio stream on open.

Audio Manager Interface with the DSP Interface

This section defines the messages that flow between the audio task 538on the audio/comm board 206 and the installable device driver on thehost processor 202. Messages to the audio task are sent usingdspPostMessage. The messages that return information from the audio taskto the host driver are delivered as callback messages.

Host Processor to Audio/Comm Board Messages

All messages from the host processor to the audio/comm board are passedin a DSPMSG structure as the dwMsg field. Additional parameters (ifused) are specified in the dwArgs DWORD array, and are called out anddefined in each of the following messages:

    ______________________________________                                        ACAPTURE.sub.-- TMSG:                                                                           Causes the audio task to start                                                or stop the flow of data from                                                 the audio source. This message                                                is a toggle (i.e., if the audio is                                            flowing, it is stopped; if it is                                              not, it is started).                                        AMUTE.sub.-- TMSG:                                                                              Toggles the codec into or takes                                               it out of muting mode.                                      APLAY.sub.-- TMSG:                                                                              Toggles playback of audio                                                     from a network source.                                      ALINKIN.sub.-- TMSG:                                                                            Connects/disconnects the audio                                                task with a virtual circuit                                                   supported by the network task.                                                The virtual circuit ID is passed                                              to the audio task in the first                                                DWORD of the dwArgs array.                                                    The virtual circuit (or channel                                               ID) is valid in both the host                                                 processor and the audio/comm                                                  board environment.                                          ALINKOUT.sub.-- TMSG:                                                                           Connects the audio task with a                                                virtual circuit supported by the                                              network task. The virtual cir-                                                cuit ID is passed to the audio                                                task in the first DWORD of                                                    the dwArgs array.                                           AREGISTERMON.sub.-- TMSG:                                                                       Registers a monitor on the                                                    specified stream. The stream                                                  ID is passed to the audio task                                                in the first DWORD of the                                                     dwArgs array, the second con-                                                 tains the notification frequency.                           APACKETNUMBER.sub.-- TMSG:                                                                      Issues a callback to the Audio                                                Manager defining the current                                                  packet number for this stream.                                                The stream ID is passed to the                                                audio task in the first DWORD                                                 of the dwArgs array.                                        ACNTL.sub.-- TMSG:                                                                              Sets the value of the specified                                               attribute on the audio device.                                                Three elements of the dwArgs                                                  array are used. The first                                                     parameter is the stream ID,                                                   the second indicates the audio                                                attribute to be adjusted, and                                                 the third is the value of the                                                 audio attribute.                                            ______________________________________                                    

Audio/Comm Board to Host Processor Messages

All messages from the audio/comm board to the host processor are passedback through the registered callback function. The message from the DSPtask to the host driver are received in the dwParam1 parameter of theregistered callback function.

Each message sent to the audio task (running on the audio/comm board)from the host processor is returned by the audio/comm board through thecallback function. Each time a message is sent to the audio/comm board,a DSPMSG is generated from the audio/comm board to respond. The messageis the same message that was sent to the board. The parameter is inDSPMSG.dwArgs[STATUS₋₋ INDEX]. This parameter is either ABOARD₋₋ SUCCESSor an error code. Error codes for each of the messages from the boardwere defined in the previous section of in this specification.

Messages that cause response to host processor action other than justsending messages (e.g., starting the audio task through the DSPinterface) are as follows:

    ______________________________________                                        AOPEN.sub.-- TMSG  Message returned in                                                           response to the device                                                        opening properly (i.e., called                                                in response to dspOpenTask).                               ASETUP.sub.-- TMSG Once the installable driver                                                   receives the                                                                  AOPEN.sub.-- TMSG from the                                                    board, it sends a data stream                                                 buffer to the task containing                                                 additional initialization infor-                                              mation (e.g., compression                                                     and SAC stream stack and                                                      initial attributes). Once the                                                 task has processed this                                                       information, it sends an                                                      ASETUP.sub.-- TMSG message                                                    to the host.                                               ACHANNEL.sub.-- HANGUP.sub.-- TMSG                                                               This message is delivered to                                                  the host when the Communi-                                                    cation subsystem notifies the                                                 task that the channel upon                                                    which it was transmitting/                                                    receiving audio samples                                                       went away.                                                 ______________________________________                                    

Wave Audio Implementation

The DSP Wave driver design follows the same architecture as the audiosubsystem (i.e., split between the host processor and the audio/commboard). For full details on the Microsoft® Wave interface, see theMicrosoft® Multimedia Programmer's Reference. Some of the controlfunctions provided by the audio manager are duplicated in the Wave/MediaControl Interface. Others, such as input gain or input and output deviceselection, are controlled exclusively by the Media control interface.

Audio Subsystem Audio/Comm Board-Resident Implementation

The audio task 538 of FIGS. 5 and 13 is actually a pair of SPOX®operating system tasks that execute on the audio/comm board 206 andtogether implement capture and playback service requests issued by thehost processor side of the audio subsystem. Referring again to FIG. 13,the audio task connects to three other subsystems running under SPOX®operating system :

1. The audio task connects to and exchanges messages with the hostprocessor side of the audio subsystem via the host device driver 536(DSH₋₋ HOST). TMB₋₋ getMessage and TMB₋₋ postMessage calls are used toreceive messages from and route messages to the audio manager 520through the host device driver 536.

2. The audio task connects to the audio hardware on the audio/comm boardvia a stream of stackable drivers terminated by the SAC device driver.This connection is bi-directional. Stackable drivers on the streamrunning from the SAC driver to the audio task include the compressiondriver and automatic gain control driver.

3. The audio task connects with comm task 540 (the board-residentportion of the comm subsystem) via a mailbox interface exchangingcontrol messages and a streams interface for exchanging data. Thestreams interface involves the use of pipe drivers. Ultimately, theinterface allows the audio task to exchange compressed data packets ofaudio samples across ISDN lines with a peer audio task running on anaudio/comm board located at the remote end of a video conference.

The audio task is composed of two SPOX® operating system tasks referredto as threads for the purposes of this specification. One thread handlesthe capture side of the audio subsystem, while the other supports theplayback side. Each thread is created by the host processor side of theaudio subsystem in response to an OpenDriver call issued by theapplication. The threads exchange compressed audio buffers with the commtask via a streams interface that involves bouncing buffers off a pipedriver. Control messages are exchanged between these threads and thecomm task using the mailbox interface which is already in place fortransferring messages between DSP tasks and the host device driver 536.

The playback thread blocks waiting for audio buffers from the comm task.The capture thread blocks waiting for audio buffers from the SAC. Whileactive, each thread checks its dedicated control channel mailbox forcommands received from the host processor as well as unsolicitedmessages sent by the comm task. A control channel is defined as the pairof mailboxes used to communicate between a SPOX® operating system taskand its DSP counterpart running on the host processor.

Audio Task Interface with Host Device Driver

The host processor creates SPOX® operating system tasks for audiocapture and playback. Among the input parameters made available to thesethreads at entry is the name each thread will use to create a stream ofstackable drivers culminating in the SAC device driver. Once the tasksare created, they send an AOPEN₋₋ TMSG message to the host processor.This prompts the host processor to deliver a buffer of additionalinformation to the task. One of the fields in the sent structure is apathname such as:

    "/tsp/gsm:0/mxr0/esp/VCadc8K"

The task uses this pathname and other sent parameters to complete itsinitialization. When finished, it sends an ASETUP₋₋ TMSG message to thehost signaling its readiness to receive additional instructions.

In most cases, the threads do not block while getting messages fromTMB₋₋ MYMBOX or posting messages to TMB₋₋ HOSTMBOX. In other words,TMB₋₋ getMessage and TMB₋₋ putMessage are called with timeout=0.Therefore, these mailboxes are preferably of sufficient depth such thatmessages sent to the Host by the threads are not dropped. ThedspOpenTask lpdspTaskAttrs "nMailboxDepth" parameter are preferably sethigher than the default value of 4. The audio task/host interface doesnot support a data channel. Thus, the "nToDsp" and "nFromDsp" fields ofdspOpenTask lpdspTaskAttrs are preferably set to 0.

Audio Task Interface with Audio Hardware

Referring now to FIG. 15, there is shown a block diagram of interfacebetween the audio task 538 and the audio hardware of audio/comm board206 of FIG. 13, according to a preferred embodiment of the presentinvention. FIG. 15 illustrates how input and output streams to the audiohardware might look after successful initialization of the capture andplayback threads, respectively.

On the capture side, audio data is copied into streams by the SAC devicedriver 1304 (the SAC). The buffer comes from a pool allocated to thisIO₋₋ SOURCE driver via IO₋₋ free() calls. The data works its way up tothe capture thread 1502 when the latter task issues an SS₋₋ get() call.The data is transformed each time it passes through a stackable driver.The mixer/splitter driver 1510 may amplify the audio signals or it maysplit the audio stream sending the second half up to the host to allowfor the recording of a video conference. The data is then compressed bythe compression driver 1508. Finally, timestamp driver 1506 appends atimestamp to the buffer before the capture thread receives it completingthe SS₋₋ get(). The capture thread 1502 either queues the bufferinternally or calls IO₋₋ free() depending on whether the capture threadis trying to establish some kind of latency or is active but unlinked),or the capture thread sends the buffer to the comm task via the pipedriver interface.

On the playback side, audio data is received in streams buffers piped tothe playback thread 1504 from the comm task. The playback threadinternally queues the buffer or flees the buffer by passing the bufferback to the pipe driver; or the playback thread calls SS₋₋ put() to sendthe buffer down the playback stream ultimately to the SAC 1304 where thesamples are played. First, the timestamp is stripped off the buffer bytimestamp driver 1506. Next, the buffer is decompressed by decompressiondriver 1508. Prior to it being played, the audio data undergoes one ormore transformations mixing in other sound or amplifying the sound(mixer/splitter driver 1510), and reducing or eliminating echoes(echo/suppression driver 1512). Once the data has been output to thesound hardware, the containing buffer is ready to be freed back up thestream satisfying an IO₋₋ alloc() issued from the layers above.

Timestamp Driver

The video manager synchronizes with the audio stream. Therefore, all theaudio task needs to do is timestamp its stream and provide an interfaceallowing visibility by the video manager into this timestamping. Theinterface for this is through the host processor requests AREGISTERMON₋₋TMSG and APACKETNUMBER₋₋ TMSG. The timestamp is a 32-bit quantity thatis initialized to 1, incremented for each block passed to the audio taskfrom the IO₋₋ SOURCE stack and added to the block. The timestamp isstripped from the block once received by the audio task executing on theremote node.

The appending and stripping of the timestamp is done by the timestampdriver 1506 of FIG. 15. Performing the stamping within a separate driversimplifies the audio task threads by removing the responsibility ofsetting up and maintaining this header. However, in order to implementthe APACKETNUMBER₋₋ TMSG host command, the threads are able to accessand interpret this header in order to determine the packet number.

On the capture side of the audio task, the capture thread will haveallocated stream buffers whose size is large enough to contain both thepacket header as well as the compressed data block. The timestamp driverdeals with each buffer as a SPOX® operating system IO₋₋ Frame data type.Before the frames are IO₋₋ free()'ed to the compression stackable driverbelow, the timestamp driver subtracts the size of the packet header fromthe frame's current size. When the frame returns to the timestamp drivervia IO₋₋ get(), the driver appends the timestamp by restoring the sizeto "maxsize" and filling the unused area with the new header. Thehandling is reversed for the playback side. Buffers received from thecomm task contain both the compressed data block and header. Thetimestamp driver strips the header by reducing "size" to "maxsize" minusthe header size.

(De)Compression Drivers

In a preferred embodiment, the DSP architecture bundles the encode anddecode functions into one driver that is always stacked between theaudio task and the host processor. The driver performs either compressor decompress functions depending on whether it is stacked within anIO₋₋ SINK or IO₋₋ SOURCE stream, respectively. Under this scheme, theaudio task only handles uncompressed data; the stackable drivercompresses the data stream on route to the host processor (IO₋₋ SINK)and decompresses the stream if data is being read from the hostprocessor (IO₋₋ SOURCE) for playback.

In an alternative preferred embodiment, the audio task deals withcompressed data in fixed blocks since that is what gets stamped orexamined on route to or from the ISDN comm task, respectively. In thisembodiment, the DSP architecture is implemented by the DXFtransformation driver 1508. Either driver may be placed in an IO₋₋SOURCE or IO₋₋ SINK stream.

Due to the audio subsystem's preference to manage latency reliably, theaudio task threads know how much capture or playback time is representedby each compressed data sample. On the capture side, this time may becalculated from the data returned by the compression driver via theDCO₋₋ FILLEXTWAVEFORMAT control command. DCO₋₋ ExtWaveFormat data fields"nSamplesPerSec" and "wBitsPerSample" may be used to calculate a buffersize that provides control over latency at a reasonable level ofgranularity.

Consider the following example. Suppose we desire to increase ordecrease latency in 50 millisecond increments. Suppose further that aDCO₋₋ FILLEXTWAVEFORMAT command issued to the compression driver returnsthe following fields: ##EQU2## If we assume that compressed samples arepacked into each 32-bit word contained in the buffer, then one TI C31DSP word contains 16 compressed samples. The buffer size containing 50ms worth of data would be: ##EQU3## this qualify, the capture threadadds the size of the packet header and uses the total in allocating asmany streams buffers as needed to service its IO₋₋ SOURCE stream.

On the receiving side, the playback thread receives the packetcontaining the buffer of compressed data. The DCO₋₋ FILLEXTWAVEFORMATcontrol command is supported by the encoder, not the decoder which theplayback thread has stacked in its IO₋₋ SINK stream. In fact, the threadhas to send the driver a DCO₋₋ SETEXTWAVEFORMAT command before it willdecompress any data. Thus, we need a mechanism for providing theplayback thread a DCO₋₋ ExtWaveFormat structure for handshaking withdecompression driver prior to entering the AST₋₋ PLAY state.

Mixer/Splitter Driver

The mixer/splitter driver 1510 (i.e., the mixer) is a stackable driverthat coordinates multiple accesses to the SAC 1304, as required byconferencing. The mixer allows multiple simultaneous opens of the SACfor both input and output and mixes the channels. The mixer alsosupports priority preemption of the control-only SAC device "sacctrl."

The SPOX® operating system image for the audio/comm board has mappingsin the device name space to transform references to SAC devices into adevice stack specification that includes the mixer. For example, a taskthat attempts to open "/sac" will actually open "/mxr1/sac". The mappingis transparent to the task. To avoid getting mapped through the mixer,an alternative set of names is provided. The alternative names consistof the standard device name prefixed with "VC". For example, to open thedevice "adc8K" without going through the mixer, a task would use thename "/VCadc8K". To obtain priority access to the SAC, the softwareopens the device "/mxr0/VCadc8K".

For output operation, the software opens the mixer with device ID 0; anyother client opens the mixer with device ID 1. Device ID 0 may be openedonly once; when it is, all other currently open channels are muted. Thatis, output to the channel is discarded. Subsequent opens of device ID 1are allowed if the sample rate matches. Device ID 1 may be opened asmany times as there are channels (other than channel 0). All opens afterthe first are rejected, if the sample rate does not match the firstopen. When more than one channel is open and not muted, the output ofall of them is mixed before it is passed on to the SAC.

For input operations, the software opens the mixer with device ID 0; anyother client opens the mixer with device ID 1. Device ID 0 may be openedonly once; when it is, if channel 1 is open, it is muted. That is, getoperations return frames of silence. Device ID 1 may be opened oncebefore channel 0 is open (yielding channel 1: normal record operation).Device ID 1 may also be opened once after channel 0 is opened (yieldingchannel 2: conference record operation). In the second case, the samplerate must match that of channel 0. Channel 1 returns data directly fromthe SAC (if it is not muted). Channel 0 returns data from the SAC mixedwith data from any output channels other than channel 0. This allows theuser to play back a recording during a video conference and have it sentto the remote participant. Channel 2 returns data from the SAC mixedwith the output to the SAC. This provides the capability of recordingboth sides of conference.

There are four control channels, each of which may be opened only once.They are prioritized, with channel 0 having the highest priority, andchannel 3 having the lowest. Only the open channel with the highestpriority is allowed to control the SAC. Non-conferencing software, whichopens "/sacctrl", is connected to channel 3, the lowest prioritychannel.

Mixer Internal Operation

For output operation, the mixer can, in theory, support any number ofoutput channels. The output channels are all equivalent in the sensethat the data from all of them is mixed to form the output sent to theSAC. However, there is one channel that is designated the main channel.The first channel opened that is not muted is the main channel. When themain channel is closed, if there are any other non-muted channels open,one of them is promoted to be the main channel. Opening channel 0(conference output) mutes any channels open at the time and channel 0cannot be muted. Thus, if channel 0 is open, it is always the mainchannel. Any open output channel that is not than the main channel iscalled an auxiliary channel.

When an IO₋₋ put operation is performed on a non-muted auxiliarychannel, the frame is placed on the channel's ready list. When an IO₋₋put operation is performed on the main channel, data from the auxiliarychannels' ready lists are mixed with the frame, and the frame is passedimmediately through to the SAC. If an auxiliary channel is not ready, itwill be ignored (and a gap will occur in the output from that channel);the main channel cannot be held up waiting for an auxiliary channel.

When an IO₋₋ put operation is performed on a muted channel, the frame isplaced directly on the channel's free list. The driver then sleeps for aperiod of time (currently 200 ms) to simulate the time it would take forthe data in the frame to be played. This is actually more time than itwould normally take for a block of data to be played; this reduces theCPU usage of muted channels.

An IO₋₋ alloc operation on the main channel is passed directly throughto the SAC; on other channels, it returns a frame from the channel'sfree list. If a frame is not available, it waits on the conditionfreeFrameAvailable. When the condition is signaled, it checks againwhether the channel is the main channel. If the main channel was closedin the meantime, this channel may have been promoted.

The mixer does not allocate any frames itself. All the frames it managesare those provided by the task by calling IO₋₋ free or IO₋₋ put. For anauxiliary channel, frames passed to IO₋₋ free are placed on thechannel's free list. These are then returned to the task when it callsIO₋₋ alloc. After the contents of a frame passed to IO₋₋ put have beenmixed with the main channel, the frame is returned to the channel's freelist. Since I/O operations on the main channel (including IO₋₋ free andIO₋₋ alloc) are passed through to the SAC, no buffer management is doneby the mixer for the main channel, and the free list and the ready listare empty. However, the mixer does keep track of all frames that havebeen passed through to the SAC by IO₋₋ free or IO₋₋ put and returned byIO₋₋ get or IO₋₋ alloc. This is done to allow for the case where themain channel is preempted by opening the priority channel. In this case,all frames that have been passed to the SAC are recalled and placed onthe mixer's free list for that channel.

Another special case is when the main channel is closed, and there isanother open non-muted channel. In this case, this other channel ispromoted to be the main channel. The frames on its ready list are passedimmediately to IO₋₋ put to be played, and the frames on its free listare passed to IO₋₋ free. These frames are, of course, counted, in casethe new main channel is preempted again.

For output mixing, a frame on the ready list of an auxiliary channel ismixed with both the main output channel and with input channel 0(conference input), if it is open. I/O operations on these two channelsare running independently, so the mixer does not know which channel willperform I/O first, or whether operations on the two will strictlyalternate, or even if they are using the same frame size. In practice,if the conference input channel is open, the main output channel isconference output, and the two use the same frame size; however, themixer does not depend on this. However, the auxiliary channel typicallywill not be using the same frame size as either of the main channels.

To handle this situation, the mixer uses two lists and two indexpointers and a flag for each channel. The ready list, where frames areplaced when they arrive, contains frames that contain data that needs tobe mixed with both the input and the output channel. When either theinput side or the output side has used all the data in the first frameon the ready list, the frame is moved to the mix list. The flag is setto indicate whether the mix list contains data for the input side or theoutput side. If the mix list is empty, both sides take data from theready list. When all the data in a frame on the mix list has been used,the frame is moved to the free list.

Mixing operations are done in units of a main-channel frame. This maytake a portion of an auxiliary channel frame or it may take parts ofmore than one. The mixing routine loops over the main channel frame.Each pass through the loop, it determines which auxiliary channel frameto mix from, takes as much data from that frame as it can, and movesthat frame to a new list if necessary. The auxiliary channel frame tomix from is either the first frame on the mix list, if it is non-emptyand the flag is set to indicate that data has not been used from thatframe yet, or the first frame on the ready list. The index, eitherinReadyIndex or outReadyIndex, specifies the first unused sample of theframe.

For example, suppose mixing is with the main input channel (conferencein), and the data for an auxiliary output channel is such that the readlist contains two flames C and D and the mix list contains two frames Aand B, wherein mixFlags equals MXR₋₋ INPUT₋₋ DATA and inReadyIndexequals 40. Assume further that the frame size on the main channel is 160words and the frame size on the auxiliary channel is 60 words.

The first time through the loop in mix₋₋ frame, the mix list is notempty, and the mix flag indicates that the data on the mix list is forthe input channel. The unused 20 samples remaining in the first frame onthe mix list are mixed with the first 20 samples of the main channelframe. inReadyIndex is incremented by 20. Since it is now equal to 60,the frame size, we are finished with the frame. The output channel isfinished with it, since it is on the mix list, so the frame is moved tothe free list and set inReadyIndex to 0.

The second time through the loop, mix₋₋ index is 20. All 60 samples aremixed out of the first frame on the mix list, and the frame is moved tothe free list.

The third time through the loop, mix₋₋ index is 80. The mix list isempty. All 60 samples are mixed out of the first frame on the readylist. Again the frame is finished, but this time it came from the readylist, so it is moved to the mix list. The mix flag is changed toindicate that the mix list now contains data for the output channel.OutReadyIndex is not changed, so the output channel will still startmixing from the same offset in the frame that it would have used if theframe had not been touched.

The fourth time through the loop, mix₋₋ index is 140. The mix list isnot empty, but the mix flag indicates that the data on the mix list isfor the output channel, so it is ignored. The remaining 20 samples aremixed from the first frame on the ready list. All the data in the framehas not been used, so it is left on the ready list; the next time aframe is processed on the main input channel, processing continues whereit left off. After mixing is complete, the ready list contains onlyframe D, the mix list contains only frame C, mixFlags equals MXR₋₋OUTPUT₋₋ DATA, and inReadyIndex equals 20.

After each step described, the data structures are completelyself-consistent. In a more typical situation, the flames on theauxiliary channel will be much larger (usually 1024 words), and only aportion of a frame will be used for each frame on the main channel.However, the processing is always similar to one or two of the foursteps described in the example.

For input operations, unlike the output channels, the three inputchannels have distinctly different semantics. The main channel is alwayschannel 0 if it is open, and channel 1 if channel 0 is not open. Channel1 will always be muted if it is open when channel 0 is opened, andcannot be opened while channel 0 is open. Channel 2 is never the mainchannel; it can be opened only while channel 0 is open, and will bemuted if channel 0 is closed.

Operation of the main channel is similar to the operation described foroutput. When IO₋₋ get or IO₋₋ free is called, the request is passed onto the SAC. For channel 0, when the frame is returned from the SAC, anyoutput ready on auxiliary output channels is mixed with it before theframe is returned to the caller.

When channel 2 (conference record) is open, output frames on channel 0(conference output) and input frames on channel 0 (conference input)(including the mixed auxiliary output) are sent to the function record₋₋frame. Record₋₋ frame copies these frames to frames allocated from thefree list for channel 2, mixes the input and output channels, and placesthe mixed frames on the ready list. When IO₋₋ get operation is performedon channel 2, it retrieves a frame from the ready list, blocking ifnecessary until one is available. If there is no frame on the free listwhen record₋₋ frame requires one, the data will not be copied, and therewill be a dropout in the recording; however, the main channel cannot beheld up waiting for the record channel.

For conference record mixing, record₋₋ frame needs to mix frames fromboth conference input and conference output into a frame for channel 2.Again, I/O operations on the conference channels are runningindependently. The mixer uses the mix list of the conference recordchannel as a holding place for partially mixed frames. readyIndexcontains the number of samples in the first frame on the mix list whichare completely mixed. The frame size contains the total number ofsamples from either channel that have been placed in the frame. Thedifference between the frame size and readyIndex is the number ofsamples that have been placed in the frame from one channel but notmixed with the other. The flag mixFlags indicates which channel thesesamples came from.

Mixing operations are done in units of a main-channel frame, as foroutput. This may take a portion of a record channel frame or it may takeparts of more than one. The mixing routine loops over the main channelframe. Each pass through the loop, it does one of the following:

1. If the mix list contains data from the other channel, mix with thefirst frame on the mix list. readyIndex indicates the place to startmixing. If the frame is now fully mixed, move it to the ready list.

2. If the mix list contains data from this channel (or equal parts fromboth channels), and there is free space in the last frame on the mixlist, copy the data into that frame. The frame size indicates the placeto start copying.

3. If neither of the above is true, allocate a new frame from the freelist and add it (empty) to the mix list. On the next iteration, case 2will be done.

To provide mutual exclusion within the mixer, the mixer uses asemaphore. Every mixer routine that manipulates any of the data for achannel first acquires the semaphore. The semaphore mechanism is verysimilar to the monitor mechanism provided by SPOX® operating system.There are two major differences: (1) a task within a SPOX® operatingsystem monitor cannot be suspended, even if a higher priority task isready to run, and (2) when a task within a SPOX® operating systemmonitor is suspended on a condition, it implicitly releases ownership ofall monitors. In the mixer, it is necessary to make calls to routineswhich may block, such as IO₋₋ alloc, while retaining ownership of thecritical region. The semaphore is released when a task waits for amixer-specific condition (otherwise, no other task would be able toenter the mixer to signal the condition), but it is not released whenthe task blocks on some condition unrelated to the mixer, such as withinthe SAC.

Echo Suppression Driver

The echo suppression driver (ESP) 1512 is responsible for suppressingechoes prevalent when one or both users use open speakers (rather thanheadphones) as an audio output device. The purpose of echo suppressionis to permit two conferencing systems 100 connected by a digital networkto carry on an audio conversation utilizing a particular microphone anda plurality of loudspeaker device choices without having to resort toother measures that limit or eliminate acoustic feedback ("coupling")from loudspeaker to microphone.

Specifically, measures obviated by the ESP include:

An audio headset or similar device to eliminate acoustic coupling.

A commercial "speakerphone" attachment that would perform the statedtask off the PC and would add cost and complexity to the user.

The ESP takes the form of innovations embedded in the context of artknown variously as "half-duplex speakerphones" or "half-duplexhands-free telephony" or "echo suppression." The ESP does not relate toart known as "echo cancellation."

The general ideas of "half-duplex hands-free telephony" are currentpractice. Electronic hardware (and silicon) exist that embody theseideas. The goal of this technology is to eliminate substantiallyacoustic coupling from loudspeaker to microphone by arranging thatsubstantial microphone gain is never coincident with substantial speakerpower output when users are speaking.

The fundamental idea in current practice is the following: Consider anaudio system consisting of a receiving channel connected to aloudspeaker and a transmitting channel connected to a microphone. Ifboth channels are always allowed to conduct sound energy freely frommicrophone to network and from network to loudspeaker, acoustic couplingcan result in which the sound emanating from the loudspeaker is receivedby the microphone and thus transmitted back to the remote station whichproduced the original sound. This "echo" effect is annoying to users atbest and at worst makes conversation between the two stationsimpossible. In order to eliminate this effect, it is preferable to placean attenuation device on each audio channel and dynamically control theamount of attenuation that these devices apply by a central logiccircuit. This circuit senses when the remote microphone is receivingspeech and when the local microphone is receiving speech. When neitherchannel is carrying speech energy, the logic permits both attenuators topass audio energy, thus letting both stations receive a certain level ofambient noise from the opposite station. When a user speaks, the logicconfigures the attenuators such that the microphone energy passesthrough to the network and the network audio which would otherwise go tothe speaker is attenuated (this is the "talk state"). When on the otherhand speech is being received from the network and the local microphoneis not receiving speech, the logic configures the attenuatorsconversely, such that the network speech is played by the speaker andthe microphone's acoustic energy is muted by the attenuator on thatchannel (this is the "listen state").

The ESP operates without a separate dedicated speakerphone circuitdevice. The ESP operates over a network featuring an audio codec that ispermitted to distort signal energies without affecting the performanceof the algorithm. The ESP effectively distributes computational overheadsuch that redundant signal processing is eliminated.

The ESP is a distributed digital signal processing algorithm. In thefollowing, the algorithm is spoken of as "distributed," meaning that twoinstantiations of it reside on the two conferencing systems connected bya digital network, and their operation is interdependent). "Frameenergy" means a mean sum of the squares of the digitized audio sampleswithin a particular time segment called a "frame."

The instantaneous configuration of the two attenuations is encoded as asingle integer variable, and the attenuations are implemented as afractional multiplier as a computational function of the variable.

In order to classify a signal as speech, the algorithm utilizes a frameenergy threshold which is computed as an offset from the mathematicalmode of a histogram in which each histogram bin represents the count offlames in a particular energy range. This threshold varies dynamicallyover time as it is recalculated. There exists a threshold for each ofthe two audio channels.

Since both stations need access to the threshold established at aparticular station (in that one station's transmit stream becomes theother station's receive stream), the threshold is shared to bothinstantiations of the algorithm as an out-of-band network signal. Thisobviates the need for both stations to analyze the same signal, andmakes the stations immune to any losses or distortion caused by theaudio codec.

The energy of a transmitted audio frame is embedded within a field ofthe communication format which carries the digitally-compressed form ofthe frame. In this way, the interactive performance of the station pairis immune from any energy distortion or losses involved in the audiocodec.

The ESP makes possible hands-free operation for video teleconferencingproducts. It is well-known that hands-free audio conversation is a muchmore natural conferencing usage model than that of an audio headset. Theuser is freed from a mechanical attachment to the PC and can participateas one would at a conference table rather than a telephone call.

Audio Task Interface with Comm Task

The interface between the audio task to the audio hardware is based onSPOX® operating system streams. Unfortunately, SPOX® operating systemstreams connect tasks to source and sink device drivers, not to eachother. Audio data are contained within SPOX® operating system arrayobjects and associated with streams. To avoid unnecessary buffer copies,array objects are passed back and forth between the comm and audiosubsystems running on the audio/comm board using SPOX® operating systemstreams and a pipe driver. The actual pipe driver used will be based ona SPOX® operating system driver called NULLDEV. Like Spectron's version,this driver simply redirects buffers it receives as an IO₋₋ SINK to theIO₋₋ SOURCE stream; no buffer copying is performed. Unlike Spectron'spipe driver, however, NULLDEV does not block the receiving task if nobuffers are available from the sending stream and discards buffersreceived from the IO₋₋ SOURCE stream if no task has made the IO₋₋ SINKstream connection to the driver. In addition, NULLDEV will not block orreturn errors to the sender. If no free buffers are available forexchange with the sender's live buffer, NULLDEV returns a previouslyqueued live buffer. This action simulates a dropped packet condition.

Setup and teardown of these pipes will be managed by a message protocolbetween the comm task and audio task threads utilizing the existing TMBmailbox architecture built into the Mikado DSP interface.

The interface assumes that the comm task is running, an ISDN connectionhas been established, and channel ID's (i.e., virtual circuit ID's) havebeen allocated to the audio subsystem by the conferencing API. Thecapture and playback threads become the channel handlers for these ID's.The interface requires the comm task first to make available to theaudio threads the handle to its local mailbox TMB₋₋ MYMBOX. This is themailbox a task uses to receive messages from the host processor. Themailbox handle is copied to a global memory location and retrieved bythe threads using the global data package discussed later in thisspecification.

Message Protocol

Like the comm task, the audio task threads use their own TMB₋₋ MYMBOXmailboxes for receiving messages from the comm task. For the purpose ofillustration, the capture thread, playback thread and comm taskmailboxes are called TMB₋₋ CAPTURE, TMB₋₋ PLAYBACK, and TMB₋₋ COMMMSG,respectively. The structure of the messages exchanged through thesemailboxes is based on TMB₋₋ Msg defined in "TMB.H" such that:

    ______________________________________                                        typedef struct TMB.sub.-- Msg {                                                  Int msg;                                                                      Uns words[TMB.sub.-- MSGLEN];                                              } TMB.sub.-- Msg;                                                             ______________________________________                                    

The messages that define this interface will be described via examples.Currently, specific message structures and constants are defined in theheader file "AS.H".

Referring now to FIG. 16, there is shown a block diagram of theinterface between the audio task 538 and the comm task 540 of FIGS. 5and 13, according to a preferred embodiment of the present invention.For audio capture, when the capture thread receives an ALinkOutTMsgmessage from the host processor, it sends an AS₋₋ REGCHANHDLR message tothe TMB₋₋ COMMMSG mailbox. The message contains an on-board channel ID,a handle to the mailbox owned by the capture thread, and a stringpointer to the pipe.

    __________________________________________________________________________    typedef struct AS.sub.-- OPENMSG {                                            Uns        msg;   /* msg == AS.sub.-- REGCHANHDLR. */                         Uns        Channel.sub.-- ID;                                                                   /* On board channel ID */                                   TMB.sub.-- MBox                                                                          mailBox;                                                                             /* Sending Task's mailbox. */                               String     DevName;                                                                             /* Device name to open. */                                  } AS.sub.-- OPENMSG;                                                          __________________________________________________________________________

Channel₋₋ ID is used to retrieve channel specific information. The taskstores this information in the global name space. A pointer to thisspace is retrieved via the routine GD₋₋ getAddress(ID). The informationhas the following structure:

    ______________________________________                                        typedef struct COMM.sub.-- AUDIO.sub.-- DATA {                                       struct                                                                              {                                                                             unsigned int   :30;                                                           unsigned int initialized                                                                     :1;                                                            unsigned int read                                                                            :1;                                                      } bool                                                                        Uns   localID;                                                                Uns   remoteID;                                                        } CommAudioData, *CommAudioDataPtr;                                           ______________________________________                                    

This structure is declared in "AS.H ". From this structure, the commtask can determine if the buffer is initialized (it always should be orthe audio tasks would not be calling), if the task is expecting to reador write data to/from the network (if read is 1, the comm task will openthe pipe for write and put data from the network there), and finally thelocal and remote IDs of the network channels.

The following pseudo code illustrates the actions performed by thecapture thread to establish a link with the comm task:

    __________________________________________________________________________    AS.sub.-- OPENMSG                                                                         *audio;                                                           TMB.sub.-- Msg                                                                            message;                                                          CommAudioDataPtr                                                                          pCAData;                                                          pCAData = (CommAudioDataPtr) GD.sub.-- getAddress(AS.sub.-- CAPTURE.sub.--     CHAN)                                                                        <set pCAData fields>                                                          audio = (AS.sub.-- OPENMSG *) &message;                                       audio->msg = AS.sub.-- REGCHANHDLR;                                           audio->Channel.sub.-- ID = (Uns) AS.sub.-- CAPTURE.sub.-- CHAN;               audio->mailBox = (TMB.sub.-- MBox) TMB.sub.-- CAPTURE;                        audio->DevName = (String) "/null";                                            TMB.sub.-- postMessage(TMB.sub.-- COMMMSG, audio, 0);                         __________________________________________________________________________

The comm task's first action will be to call GD₋₋ getAddress() andretrieve an address to the CommAudioData structure. It validates thestructure using the local and remote IDs linking the thread with theappropriate ISDN channel. Finally, the comm task responds by connectingto its end of audio>DevName ("/null") and returning status to thecapture thread via a message directed to TMB₋₋ CAPTURE such that:

    __________________________________________________________________________    TMB.sub.-- Msg                                                                            message;                                                          CommAudioDataPtr                                                                          pCAData;                                                          AS.sub.-- OPENMSG                                                                         audio;                                                            typedef struct AS.sub.-- INFOMSG {                                            Uns     msg;    /* AS.sub.-- CLOSE.sub.-- CHAN or AS.sub.-- STATUS */         Uns     Channel.sub.-- ID;                                                                    /* On board channel ID */                                     Uns     statusCode;                                                                           /* Status Code */                                             Uns     statusExtra;                                                                          /* Additional status info */                                  } AS.sub.-- INFOMSG *comm;                                                    TMB.sub.-- getMessage (TMB.sub.-- COMMMSG, (TMB.sub.-- Msg)&audio, 0);        pCAData = (CommAudioDataPtr) GD.sub.-- getAddress(audio.Channel.sub.--        ID);                                                                          <validate pCAData fields and open audio.DevName>                              comm = (AS.sub.-- INFOMSG *) &message;                                        comm->msg = AS.sub.-- STATUS;                                                 comm->Channel.sub.-- ID = audio.Channel.sub.-- ID;                            comm->statusCode = AS.sub.-- REGCHANHDLR.sub.-- OK;                           TMB.sub.-- postMessage (audio.mailbox, comm, 0);                              __________________________________________________________________________     If the comm task detects an error, the statusCode and statusExtra fields     are set to the appropriate error codes defined in the section Status and     Error Codes.

The capture thread subsequently receives stream buffers filled with timestamped and compressed audio data from the input driver stack via SS₋₋get() calls and routes them to the comm task via the pipe driver. Aftereach SS₋₋ put() to the pipe driver, the capture thread notifies the commtask that an incoming buffer is on the way via an AS₋₋ RECEIVECOMPLETEstatus message.

audio=(AS₋₋ INFOMSG *) &message;

audio→msg=AS₋₋ STATUS;

audio→Channel₋₋ ID=AS₋₋ CAPTURE₋₋ CHAN;

audio→statusCode=AS₋₋ RECEIVECOMPLETE;

TMB₋₋ postMessage (TMB₋₋ COMMMSG, audio, 0);

The comm task sends the buffers to the ISDN driver which transmits thedata frame on the audio output's ISDN virtual channel.

Between each input streams buffer processed, the capture thread checksTMB₋₋ CAPTURE for new requests messages from the comm task or the hostprocessor. When a second ALINKOUT₋₋ TMSG message is received from thehost processor, the capture thread stops sending data buffers to thepipe driver and notifies the comm task of its intention to terminate thelink:

audio=(AS₋₋ INFOMSG *) &message;

audio→msg=AS₋₋ CLOSE₋₋ CHAN;

audio→Channel₋₋ ID=AS₋₋ CAPTURE₋₋ CHAN;

TMB₋₋ postMessage (TMB₋₋ COMMMSG, audio, 0);

Capture treats the ALINKOUT₋₋ TMSG message as a toggle: the firstreceipt of the message establishes the link, the second receiptterminates it. The comm task first closes its half of the pipe driverand then terminates its connection with the capture thread via an AS₋₋CLOSE₋₋ CHAN₋₋ OK message.

comm→msg=AS₋₋ STATUS;

comm→Channel₋₋ ID=Channel₋₋ ID;

comm→statusCode=AS₋₋ CHANCLOSE₋₋ OK;

TMB₋₋ postMessage (TMB₋₋ CAPTURE, comm, 0);

On the other side of the audio task, the playback thread waits for theALINKIN₋₋ TMSG message from the host processor after first opening theIO₋₋ SINK side of a second pipe driver "/null2". When that messagefinally arrives, the playback thread opens the communication pathway tothe comm task and registers as the audio input channel handler via anAS₋₋ REGCHANHDLR message. Like the capture thread, the playback threadsupplies the channel ID, its response mailbox, and a string pointer tothe second pipe driver:

pCAData=(CommAudioDataPtr) GD₋₋ getAddress(AS₋₋ PLAYBACK₋₋ CHAN) <setpCAData fields>

audio=(AS₋₋ OPENMSG *) &message;

audio→msg=AS₋₋ REGCHANHDLR;

audio→Channel₋₋ ID=(Uns) AS₋₋ PLAYBACK₋₋ CHAN;

audio→mailbox=(TMB₋₋ MBox) TMB₋₋ PLAYBACK;

audio→DevName=(String) "/null2";

TMB₋₋ postMessage (TMB₋₋ COMMMSG, audio, 0);

Exactly as with the capture thread, the comm task behaves as follows:

TMB₋₋ getMessage (TMB₋₋ COMMMSG, (TMB₋₋ Msg)&audio, 0);

pCAData=(CommAudioDataPtr) GD₋₋ getAddress(audio.Channel₋₋ ID);

<validate pCAData fields and open audio.DevName>

comm=(AS₋₋ INFOMSG *) &message;

comm→msg=AS₋₋ STATUS;

comm→Channel₋₋ ID=audio.Channel₋₋ ID;

comm→statusCode=AS₋₋ REGCHANHDLR₋₋ OK;

TMB₋₋ postMessage (audio.mailbox, comm, 0);

Once this response is received, the playback thread blocks waiting fornotification of input buffers delivered by the comm task to its side thepipe driver. After each buffer is put to pipe, the comm task notifiesthe playback thread:

comm=(AS₋₋ INFOMSG *) &message;

comm→msg=AS₋₋ STATUS;

comm→Channel₋₋ ID=Channel₋₋ ID;

comm→statusCode=AS₋₋ RECEIVECOMPLETE;

TMB₋₋ postMessage (TMB₋₋ PLAYBACK, comm, 0);

The playback thread collects each buffer and outputs the audio data bySS₋₋ put()'ing each buffer down the driver stack to the SAC 1304.

The handing of the second ALINKIN₋₋ TMSG request received from the hostprocessor is the same as on the capture side. The playback thread closes"/null2" and uses AS₋₋ CLOSE₋₋ CHAN to sever its link with the commtask.

At any time during the link state, problems with or a normal shutdown ofthe ISDN logical channel may generate a hang-up condition. The comm tasknotifies the capture and/or playback thread via the unsolicited statusmessage AS₋₋ COMM₋₋ HANGUP₋₋ NOTIFY:

comm=(AS₋₋ INFOMSG *)&message;

comm→msg=AS₋₋ STATUS;

comm→Channel₋₋ ID=Channel₋₋ ID;

comm→statusCode=AS₋₋ COMM₋₋ HANGUP₋₋ NOTIFY;

comm→statusExtra=<QMUX error>

TMB₋₋ postMessage (<TMB₋₋ PLAYBACK or TMS₋₋ CAPTURE >, comm, 0);

In response, the threads close the channel, notifying the host processorin the process.

As defined in "AS.H", the following are status and error codes for thestatusCode field of AS₋₋ STATUS messages:

    __________________________________________________________________________    AS.sub.-- REGCHANHDLR.sub.-- OK                                                                 AS.sub.-- REGCHANHDLR request succeeded.                    AS.sub.-- REGCHANHDLR.sub.-- FAIL                                                               AS.sub.-- REGCHANHDLR request failed.                       AS.sub.-- CHANCLOSE.sub.-- OK                                                                   AS.sub.-- CHANCLOSE request succeeded.                      AS.sub.-- CHANCLOSE.sub.-- FAIL                                                                 AS.sub.-- CHANCLOSE request failed.                         AS.sub.-- COMM.sub.-- HANGUP.sub.-- NOTIFY                                                      Open channel closed.                                        AS.sub.-- RECEIVECOMPLETE                                                                       Data packet has been sent to NULLDEV.                       AS.sub.-- LOST.sub.-- DATA                                                                      One or more data packets dropped.                           __________________________________________________________________________

Regarding buffer management issues, the audio task maintain adynamically configurable amount of latency on the audio streams. To dothis, both audio task threads have control over the size of the buffersthat are exchanged with the comm task. As such, the comm task adopts thebuffer size for the streams assigned it by the audio task. In addition,the number of buffers which exist within the NULLDEV link between thecomm task and an audio task thread are defined by the threads.Mechanisms for implementing this requirement involves the followingsteps:

1. Both audio task threads create their SPOX® operating system streamconnections to the NULLDEV pipe driver before registering with the commtask. Each thread issues an SS₋₋ create() specifying the buffer sizeappropriate for the audio compression method and time stamp framing tobe performed on each buffer. In addition, the attrs.nbufs field is setto the desired number of buffers available for queuing audio data withinthe NULLDEV link.

2. When setting up its NULLDEV streams, the comm task sets the SS₋₋create() buffer size parameter to -1 specifying that a "device-dependentvalue will be used for the stream buffer size". See SPECTRON's SPOX®Application Programming Reference Manual, Version 1.4, page 173. Inaddition, the attrs.nbufs are set to 0 ensuring that no additionalbuffers are added to the NULLDEV link.

3. After opening the stream, the comm task will query for the correctbuffer size via an SS₋₋ sizeof() call. Thereafter, all buffers itreceives from the capture thread and all buffers it delivers to theplayback thread are this size. It uses this size when creating the SA₋₋Array object used to receive from and send buffers to NULLDEV.

The comm task preferably performs no buffering of live audio data.Communication between audio task endpoints is unreliable. Because audiodata is being captured, transmitted, and played back in real time, it isundesirable to have data blocks retransmitted across an ISDN channel.

Whether unreliable transmission is supported or not for the audiostream, the NULLDEV driver drops data blocks if live buffers back up.NULLDEV does not allow the sender to become buffer starved. It continuesto exchange buffers with the task issuing the SS₋₋ put(). If no freebuffers are available to make the exchange, NULLDEV returns the livebuffer waiting at the head of its ready queue.

Global Data Package

The SPOX® operating system image for the audio/comm board contains apackage referred to as the Global Data Package. It is a centralizedrepository for global data that is shared among tasks. The interfaces tothis package are defined in "GD.H". The global data is contained in aGBLDATA struct that is defined as an array of pointers:

    ______________________________________                                        typedef struct GBLDATA {                                                        Ptr availableData[MAX.sub.-- GLOBALS];                                      } GBLDATA;                                                                    ______________________________________                                    

Like all SPOX® operating system packages, the global data packagecontains an initialization entry point GD₋₋ init() that is called duringSPOX® operating system initialization to set the items in GBLDATA totheir initial values. Tasks that wish to access the global data willcontain statements like the following to obtain the contents of theGBLDATA structure:

Ptr pointerToGlobalObject;

pointerToGlobalObject=GD₋₋ getAdress(OBJECT₋₋ NUMBER);

In a preferred embodiment, there is no monitor or semaphore associatedwith the global data. So by convention, only one task will write to anitem and all others will only read it. For example, all data pointersare set to NULL by GD₋₋ init(). A pointer such asavailableData[CommMBox] would then be filled in by the comm task duringits initialization with the following sequence:

pointerToGlobalData=GD₋₋ getAddress(AS₋₋ COMMMBOX);

pointerToGlobalData→CommMBox=TMB₋₋ MYMBOX;

Tasks that wish to communicate to the comm task can check that the taskis present and obtain its mailbox handle as follows:

    __________________________________________________________________________    pointerToGlobalData = GD.sub.-- getAddress(AS.sub.-- COMMMBOX);               if (pointerToGlobalData->CommMBox != NULL) {                                         /* COMMTASK is present */                                                     TMB.sub.-- postMessage (                                                                 pointerToGlobalData->CommMBox,                                                aMessage,                                                                     timeOutValue);                                              else {                                                                               /* IT IS NOT */                                                        }                                                                             __________________________________________________________________________

NULLDEV Driver

The SPOX® operating system image for the audio/comm board contains adevice driver that supports interprocess communication though the stream(SS) package. The number of distinct streams supported by NULLDEV iscontrolled by a defined constant NBRNULLDEVS in NULLDEV.H. Currently,NULLDEV supports two streams. One is used for the audio task capturethread to communicate with the comm task. The other is used by theplayback thread to communicate with the comm task. The assignment ofdevice names to tasks is done by the following two constants inASTASK.H:

    ______________________________________                                        #define AS.sub.-- CAPTURE.sub.-- PIPE                                                               "/null"                                                 #define AS.sub.-- PLAYBACK.sub.-- PIPE                                                              "/null2"                                                ______________________________________                                    

Support for additional streams may be obtained by changing theNBRNULLDEVS constant and recompiling NULLDVR.C. The SPOX® operatingsystem config file is also adjusted by adding additional device namestrings to this section as follows:

    ______________________________________                                               driver NULLDEV.sub.-- driver {                                                     "/null":                                                                             devid = 0;                                                             "/null2":                                                                            devid = 1;                                                        };                                                                     ______________________________________                                    

The next device is the sequence has devid=2.

SS₋₋ get() calls to NULLDEV receive an error if NULLDEV's ready queue isempty. It is possible to SS₋₋ put() to a NULLDEV stream that has notbeen opened for SS₋₋ get() on the other end. Data written to the streamin this case is discarded. In other words, input live buffers are simplyappended to the free queue. SS₋₋ put() never returns an error to thecaller. If no buffers exist on the free queue for exchange with theincoming live buffer, NULLDEV removes the buffer at the head of theready queue and returns it as the free buffer.

Comm Subsystem

The communications (comm) subsystem of conferencing system 100 of FIG. 5comprises comm API 510, comm manager 518, and DSP interface 528 runningon host processor 202 of FIG. 2 and comm task 540 running on audio/commboard 206. The comm subsystem provides connectivity functions to theconferencing application programs 502 and 504. It maintains and managesthe session, connection, and the virtual channel states. All theconnection control, as well as data communication are done through thecommunication subsystem.

Referring now to FIG. 17, there is shown a block diagram of the commsubsystem of conferencing system 100 of FIG. 5, according to a preferredembodiment of the present invention. The comm subsystem consists of thefollowing layers that reside both on host processor 202 and theaudio/comm board 206:

Transport independent interface (TII.DLL),

Reliable datalink module (DLM.DLL+KPDAPI.DLL, where KPDAPI.DLL is theback-end of the DLM which communicates with the DSP interface), and

Datalink module.

TII.DLL and RDLM.DLL reside entirely on the host processor. Datalinkmodule comprises DLM.DLL residing on the host processor, and control (Dchannel), D channel driver, data comm tasks, and B channel driversresiding on audio/comm board 206.

The comm interface provides a "transport independent interface" for theconferencing applications. This means that the comm interface hides allthe network dependent features of the conferencing system. In apreferred embodiment, conferencing system 100 uses the ISDN Basic RateInterface (BRI) which provides 2*64 KBits/sec data (B) channels and onesignaling (D) channel (2B+D) . Alternative preferred embodiment may usealternative transport media such as local area networks (LANs) as thecommunication network.

Referring now to FIG. 18, there is shown a block diagram of the commsubsystem architecture for two conferencing systems 100 participating ina conferencing session, according to a preferred embodiment of thepresent invention. The comm subsystem provides an asynchronous interfacebetween the audio/comm board 206 and the conferencing applications 502and 504.

The comm subsystem provide all the software modules that manage the twoISDN B channels. The comm subsystem provides a multiple virtual channelinterface for the B channels. Each virtual channel is associated withtransmission priority. The data queued for the higher priority channelsare transmitted before the data in the lower priority queues. Thevirtual channels are unidirectional. The conferencing applications openwrite-only channels. The conferencing applications acquire read-onlychannels as a result of accepting a open channel request from the peer.The DLM supports the virtual channel interface.

During a conferencing session, the comm subsystem software handles allthe multiplexing and inverse multiplexing of virtual channels over the Bchannels. The number of available B channels (and the fact that there ismore than one physical channel available) is not a concern to theapplication.

The comm subsystem provides the D channel signaling software to the ISDNaudio/comm board. The comm subsystem is responsible for providing theISDN B channel device drivers for the ISDN audio/comm board. The commsubsystem provides the ISDN D channel device drivers for the ISDNaudio/comm board. The comm software is preferably certifiable in NorthAmerica (U.S.A., Canada). The signaling software is compatible with NI1,AT&T Custom, and Northern Telecom DMS-100.

The comm subsystem provides an interface by which the conferencingapplications can gain access to the communication hardware. The goal ofthe interface is to hide the implementation of the connectivitymechanism and provide an easy to use interface. This interface providesa very simple (yet functional) set of connection control features, aswell as data communication features. The conferencing applications usevirtual channels for data communication. Virtual channels are simplex,which means that two virtual channels are open for full duplexcommunication between peers. Each conferencing application opens itsoutgoing channel which is write-only. The incoming (read-only) channelsare created by "accepting" an "open channel" request from the peer.

qMUX MULTIPLE CHANNEL STREAMING MODULE

The QSource Multiple Channel Streaming Module (qMUX) is based on theneed to utilize the high bandwidth of two bearer (B) channels (each at64 kbps) as a single high-speed channel for the availability of multipleupper layer users. This section specifies the various interfaces betweenQSource qMUX module and other QSource modules or application modules toachieve this objective.

QSource qMUX is a data link provider for one or more end-to-endconnected upper layers to exchange data between themselves at a higherdata rate than is possible over a single bearer (B) channel. qMUXaccepts messages from upper layer providers and utilizes both B channelsto transfer the data. On the receiving end, qMUX will reassemblereceived buffers from Layer 1 in sequential order into a user messageand deliver the message to the awaiting upper layer. There is no dataintegrity insured by qMUX. There is no Layer 2 protocol (i.e., LAPB)used in the transmission of packets between the two endpoints; however,packets are transmitted using HDLC framing. Throughout this section, theterm ULP means Upper Layer Process or qMUX User.

qMUX is a data link provider process that receives user data frames fromupper layers (data link user) and equally distributes them over the twoB channels. This achieves a higher bandwidth for an upper layer than ifa single B channel was used. Several higher processes can be multiplexedthrough the qMUX process, each being assigned its own logical channelthrough qMUX. This logical channel is known as a qMUX logical identifier(qLI).

A priority is assigned to each qLI as it is opened. This priorityensures that buffers of higher priority are sent before buffers oflesser priority are transmitted over the B channels. This enables anupper layer, whose design ensures a smaller bandwidth usage, to behandled in a more timely manner, ensuring a more rapid exchange of databetween the two end users.

qMUX is an unreliable means of data transfer between two end users.There is no retransmission of message data. Although received packetsare delivered to the higher requesting layers, there is no guarantee ofdata integrity maintained between the two cooperating qMUX processes.Packets may be lost between the two endpoints because there is no Layer2 protocol (i.e., LAPB) used in the transmission of packets between thetwo end-points; however, packets are transmitted using HDLC flaming. Inorder to provide reliability, a transport provider such as TP0 (modifiedto work with qMUX) is preferably used as a ULP. qMUX considers a messageas one or more data buffers from the higher layer. These chained buffersare unchained, assigned sequence numbers within the message sequence,and transferred to the far end. Each buffer contains a sequence numberthat reflects its place within the message.

At the receiving end, the buffers are reassembled into messages anddelivered to the awaiting upper layer. Message integrity is notguaranteed. Messages are discarded on the receiving end if buffers arenot received before final reassembly and delivery.

All messages transmitted by qMUX are preferably split into an evennumber of buffers, independent of message size. Two processes, namelySM2 and SCUD, split messages into equal buffers. In an alternativepreferred embodiment, messages are split after exceeding a specific size(160 octets). Splitting messages into an even number of buffers,regardless of size, ensures timely delivery of data. In anotheralternative preferred embodiment, qMUX transmits a message contained ina single buffer.

Upper layers ensure that both endpoints are synchronized on their qLI(logical channel identifier) and priority. Once both B channels areestablished, the ULP establishes a qMUX logical interface with the qMUXprocess. This qLI, assigned by the ULP, allows for the transfer of databetween qMUX and the ULP. This qLI assignment may be transferred orreassigned to another ULP, by use of the qMUX₋₋ BIND₋₋ REQUESTprimitive. The qLI may be used by only one ULP at a time. The maximumqLI value in a system is defined as a startup parameter (MAX₋₋ LOGICAL₋₋CHANNELS). A ULP requesting a qLI when all of the assignable qLI are inuse is denied.

If a message is received for a qLI that is not assigned, then themessage is discarded. A received message has the sending qLI and theintended receiver's qLI contained in the message. If the ULP assigned tothe qLI does not have an outstanding request to receive data when amessage is received, the message is discarded as well.

A qLI of 0 (zero) is used as a control channel for a ULP requestingassignment as a controlling ULP. The controlling qLI may be used tosynchronize the two end ULPs cooperating in the data exchange.

When a qLI is requested, the requesting ULP assigns a priority for thehandling of messages. Those ULPs requiring a high throughput with verylittle bandwidth should request a high priority to its messages.Priority is valid for outgoing messages only; that is, the priority isused when the buffer is queued to the B channel driver.

Data transfer between the ULP and qMUX is performed on a message basis.A message is defined to be one or more data buffers containing userdata. The buffers are dis-assembled, assigned sequence numbers, andtransferred over the available bandwidth of the two B channels in theirassigned priority order, and re-assembled on the far-end for delivery toa requesting ULP. Should a fragment of the message not be delivered, theentire message is discarded; no retransmission of the message or its-parts are attempted by qMUX.

End-to-End flow control is not performed by qMUX. Before buffers arequeued to layer 1, the queue depth is checked. If the number of bufferson a B-channel queue exceeds 15, the message is discarded, andnotification given to the ULP.

qMUX maintains a message window per qLI that effectively buffersincoming messages. This guards against network transit delays that mayexist due to the two bearer channels in use. The current size of themessage window is three. For example, it is possible for qMUX to havecompletely assembled message numbers 2 and 3, while waiting for thefinal part of message 1. When message 1 is completely assembled, allthree are then queued, in message order, to the appropriate ULP. If anypart of message 4 is received before message 1 is complete, message 1 isdiscarded and the ULP notified. The message window then slides toinclude messages 2, 3, and 4. Since messages 2 and 3 are complete, theyare forwarded to the ULP and the window slides to message 4.

The following primitives are sent from the ULP to qMUX:

    ______________________________________                                        qMUX.sub.-- DATA.sub.-- REQUEST                                                                  Indicates the message carries                                                 application data. The                                                         message is comprised of one                                                   or more QSource system                                                        buffers.                                                   qMUX.sub.-- ATTACH.sub.-- REQUEST                                                                A request by a ULP for a                                                      qLI assignment. Both B                                                        channels are assumed to be                                                    connected at this time; the                                                   state of the two B channels is                                                unaltered. This request can                                                   also be used to request a                                                     controlling qLI (0) for a                                                     ULP.                                                       qMUX.sub.-- BIND.sub.-- REQUEST                                                                  A request by a ULP to have                                                    the specified qLI bound to                                                    the requesting ULP. All sub-                                                  sequent received traffic is                                                   directed to the requesting                                                    ULP.                                                       qMUX.sub.-- DEATTACH.sub.-- REQUEST                                                              Used by a ULP to end its                                                      usage of a qLI. All sub-                                                      sequent messages received                                                     are discarded for this qLI.                                                   This is used by a ULP to end                                                  the logical connection and                                                    reception of data.                                         ______________________________________                                    

The following primitives are sent from qMUX to the ULP:

    ______________________________________                                        qMUX.sub.-- DATA.sub.-- INDICATION                                                              Indicates that user data is con-                                              tained in the message. The                                                    message is one or more                                                        QSource system buffers.                                     qMUX.sub.-- OK.sub.-- ACK                                                                       Acknowledges to the ULP that                                                  a previously received primitive                                               was received successfully. The                                                qLI is returned within the                                                    acknowledgement.                                            qUMX.sub.-- ERROR.sub.-- ACK                                                                    Informs the ULP that a pre-                                                   viously issued request was                                                    invalid. The primitive in error                                               and the associated qLI (if                                                    valid) are conveyed back to                                                   the ULP.                                                    ______________________________________                                    

The following primitives are exchanged between PH (B channel Driver) and

    ______________________________________                                        PH.sub.-- DATA.sub.-- REQUEST                                                                 Used to request that the user data                                            contained in the QSource system                                               buffer be transmitted on indicated                                            B channel.                                                    PH.sub.-- DATA.sub.-- INDICATION                                                              Used to indicate to QMUX that the                                             user data in the QSource system                                               buffer is intended for an ULP.                                                This particular buffer may only be                                            a part of a message.                                          ______________________________________                                    

The following example of the usage of qMUX by two cooperating ULPs(referred to as ULP-A and ULP-B) assumes that a connection has alreadybeen established:

The session manager sends a QMUX₋₋ CONNECT₋₋ REQ primitive to qMUX thatstates that both B-channels are available. ULP-A and ULP-B establishboth B Channels at their respective ends.

ULP-A issues a qMUX₋₋ ATTACH₋₋ REQUEST for a controlling qLI to qMUX,and two qMUX₋₋ ATTACH₋₋ REQUESTs for a data exchange path. The firstpath is for sending and the second is for receiving data.

ULP-B also issues a qMUX₋₋ ATTACH₋₋ REQUEST for a controlling qLI (ofzero) to qMUX, and two qMUX₋₋ ATTACH₋₋ REQUESTs for a data exchangepath. ULP assigns zero for the controlling qLI requests and qLI 5 and 6for ULP-A and qLI 5 and 6 for LP-B.

ULP-A formats a peer-to-peer (ULP-A to ULP-B) request for informingULP-B that messages for ULP-A should be directed over qLI 6. ULP-A sendsthe message via qMUX over the controlling qLI.

ULP-B also formats a peer-to-peer (ULP-B to ULP-A) request for informingULP-A that messages for ULP-B should be directed over qLI 6. ULP-B sendsthe message via qMUX over the controlling qLI.

ULP-A receives the request from ULP-B from the controlling qLI. Aresponse is formatted which gives the qLI for ULP-A as 6 and ULP-B as 6.It is sent to qMUX for transfer over the controlling qLI.

ULP-B receives the request from ULP-A from the controlling qLI. Aresponse is formatted which gives the qLI for ULP-B as 6 and ULP-A as 6.It is sent to qMUX for transfer over the controlling qLI.

Once both ULP peers have received the responses to their peer-to-peerrequests, they an exchange data.

The following scenario illustrates the interface and design of qMUX forthe exchange of data/video/audio:

ULP-A issues a qMUX₋₋ DATA₋₋ REQUEST over qLI 5 for delivery at thefar-end to qLI 6. The message was segmented into two QSource systembuffers by SM2/SCUD and sent to the B channels as follows:

Segment one: marked as START₋₋ OF₋₋ MESSAGE, sending qLI is 5, receivingqLI is 6, sequence number is 1 (one). It is sent to the B channel driverfor B channel 1 with a primitive of PH₋₋ DATA₋₋ REQ.

Segment two: marked as END₋₋ OF₋₋ MESSAGE, sending qLI is 5, receivingqLI is 6, sequence number is 2 (two). It is sent to the B channel driverfor B channel 2 with a primitive of PH₋₋ DATA₋₋ REQ.

qMUX at the receiving end receives the buffers as follows:

Segment one: received from B channel driver on B channel 1. Buffer hasheader of START₋₋ OF₋₋ MESSAGE, sequence number 1. State is nowAWAITING₋₋ EOM for qLI 6.

Segment two: END₋₋ OF₋₋ MESSAGE received. Buffer is chained to buffertwo. Primitive is made qMUX₋₋ DATA₋₋ INDICATION and sent to the ULP-Bwho had bound itself to qLI 6. State is now set to AWAITING₋₋ START₋₋OF₋₋ MESSAGE.

The above activity occurs during the message window for this qLI. Themessage window is currently set at three. A message window exists on aqLI basis.

Comm API

Comm API 510 of FIG. 5 provides an interface between conferencingapplications 502 and 504 and the comm subsystem. Comm API 510 consistsof a transport-independent interface (TII.DLL of FIG. 17). The TIIencapsulates the network driver routines provided to the upper-layermodules (ULMs).

Comm API 510 provides the following services and functions:

Initialization Commands

BeginSession: Begins a comm session. Only one "thread" of execution isallowed to begin the comm session for a given media. This threadspecified the session handler, which is the focal point of all theconnection management events. All connection related events are given tothe session handler.

EndSession: Ends a comm session.

Connection Control Commands

MakeConnection: Makes connection to a remote peer. A MakeConnectioncommand sends a connection request to the session handler of thespecified "address".

CloseConnection: Closes a connection. This command closes all the openvirtual channels and the connection. All the relevant handlers arenotified of the events caused by this command.

AcceptConnection: Accepts a peer's request for connection. The sessionhandler of the application which has received a connection requestissues this command, if it wants to accept the connection.

RejectConnection: Rejects a peer's request for connection.

Virtual-Channel Management

RegisterChanMgr: Registers the piece of code that will handle channelevents. This call establishes a channel manager. The job of channelmanager is to field the "open channel" requests from the connected peer.

RegisterChanHandler: Registers the piece of code that will handle dataevents. The channel handler is notified of the data related events, suchas receipt of data and completion of sending of a data buffer.

OpenChannel: Opens a virtual channel for sending data.

AcceptChannel: Accepts a virtual channel for receiving data.

RejectChannel: Rejects the virtual channel request.

CloseChannel: Closes an open channel.

"Data" exchange

SendData: Sends data over a virtual channel.

ReceiveData: Posts buffers for incoming data over a virtual channel.

Communications Statistics

GetChanInfo: Returns information about a given channel (e.g., thereliability and priority of the channel).

GetChanStats: Returns statistical information about a given channel(e.g., number of transmissions, receives, errors).

GetTiiStats: Returns statistical information about the current TIIchannels.

Transport-Independent Interface

Comm API 510 supports calls to three different types oftransport-independent interface functions by conferencing applications502 and 504 to the comm subsystem: connection management functions, dataexchange functions, session management, and communications statisticsfunctions. Connection management functions provide the ULM with theability to establish and manage virtual channels for its peers on thenetwork. Data exchange functions control the exchange of data betweenconferencing systems over the network. Communications statisticsfunctions provide information about the channels (e.g., reliability,priority, number of errors, number of receives and transmissions). Thesefunctions are as follows:

    ______________________________________                                        Connection Management Functions                                               RegisterChanMgr                                                                            Registers a callback or an application                                        window whose message processing                                               function will handle low-level notifica-                                      tions generated by data channel initiali-                                     zation operations. This function is invoked                                   before any OpenChannel calls are made.                           RegisterChanHandler                                                                        Registers a callback or an application                                        window whose message processing                                               function will handle low-level notifica-                                      tions generated by data channel input/                                        output (I/O) activities. The channels that                                    are opened will receive                                                       CHAN.sub.-- DATA.sub.-- SENT, and the accepted                                channels will receive                                                         CHAN.sub.-- RECV.sub.-- COMPLTE.                                 OpenChannel  Requests a sub-channel connection from                                        the peer application. The result of the                                       action is given to the application by                                         invoking the callback routine specified in                                    the RegisterChanHandler. The application                                      must specify an ID for this transaction.                                      This ID is passed to the callback routine                                     or posted in a message.                                                       Note: All Connection requests are for                                         establishing connections for sending data.                                    The receive channels are opened as the                                        result of accepting a ConnectChannel                                          request.                                                         AcceptChannel                                                                              A peer application can issue                                                  AcceptChannel in response to a                                                CHAN.sub.-- REQUEST (OpenChannel)                                             message that has been received. The                                           result of the AcceptChannel call is a                                         one-way communication sub-channel for                                         receiving data. Incoming data notification                                    will be sent to the callback or window                                        application (via PostMessage) to the                                          ChannelHandler.                                                  RejectChannel                                                                              Rejects an OpenChannel request                                                (CHAN-REQUEST message) from the                                               peer.                                                            CloseChannel Closes a sub-channel that was opened by                                       AcceptChannel or ConnectChannel.                                 Data Exchange Functions                                                       SendData     Sends data. Data is normally sent via this                                    mechanism.                                                       ReceiveData  Receives data. Data is normally received                                      through this mechanism. This call is                                          normally issued in response to a                                              DATA.sub.-- AVAILABLE message.                                   Communications Statistics Functions                                           GetChanInfo  Returns channel information.                                     GetChanStats Returns various statistical information                                       about a channel.                                                 GetTiiStats  Returns various statistical information                                       about a TII channel.                                             ______________________________________                                    

These functions are defined in further detail later in thisspecification in a section entitled "Data Structures, Functions, andMessages."

In addition, comm API 510 supports three types of messages and callbackparameters returned to conferencing applications 502 and 504 from thecomm subsystem in response to some of the above-listed functions:session messages, connection messages, and channel messages. Sessionmessages are generated in response to change of state in the session.Connection messages are generated in response to the variousconnection-related functions.

Message and Callback Parameters

This section describes the parameters that are passed along with themessages generated by the communication functions. The events arecategorized as follows:

    ______________________________________                                        Connection Events:                                                                        Connection-related messages that are sent                                     to the session handler (e.g., connection                                      request, connection accepted, connection                                      closed).                                                          Channel Events:                                                                           Channel-related messages that are handled                                     by the channel manager (e.g., channel                                         request, channel accepted, channel closed).                       Data Events:                                                                              Events related to data communication (e.g.,                                   data sent, receive completed). These                                          events are handled by the channel handlers.                                   Each virtual channel has a channel handler.                       ______________________________________                                    

Session Handler Messages

The following messages are generated in response to the variousconnection related functions:

    ______________________________________                                        CONN.sub.-- REQUESTED                                                         wParam        Connection handle                                               lparam        Pointer to incoming connection                                                information structure:                                                        {                                                                           WORD       Session handle                                                     LPTADDR    Pointer to caller's                                                           address                                                            LPCONN.sub.-- CHR                                                                        Pointer to connec-                                                            tion attributes                                                    }                                                                 CONN.sub.-- ACCEPTED                                                                        Response to MakeConnection or                                                 AcceptConnection request.                                       wParam        Connection handle                                               lParam        Pointer to connection information                                             structure:                                                                    {                                                                           DWORD      TransId (specified                                                            by user in earlier                                                            request)                                                           LPCONN.sub.-- CHR                                                                        Pointer to connec-                                                            tion attributes                                                    }                                                                 CONN.sub.-- REJECTED                                                                        Response to MakeConnection request.                             wParam        Reason                                                          lParam        TransId (specified by application                                             in earlier request)                                             CONN.sub.-- TIMEOUT                                                                         Response to MakeConnection                                                    request).                                                       lParam        TransId (specified by application                                             in earlier request)                                             CONN.sub.-- ERROR                                                                           Indication of connection closed                                               due to fatal error.                                             wParam        Connection handle                                               lParam        Error                                                           CONN.sub.-- CLOSED                                                                          Indication of remote Close.                                     wParam        Connection handle                                               CONN.sub.-- CLOSE.sub.-- RESP                                                               Response to CloseConnection request.                            wParam        Connection handle                                               lParam        TransId (specified by application                                             in earlier Close request)                                       SESS.sub.-- CLOSED                                                                          Response to EndSession request.                                 wParam        Session handle                                                  ______________________________________                                    

Channel Manager Messages

The following messages are generated in response to the various channelmanagement functions as described with the function definitions:

    ______________________________________                                        CHAN.sub.-- REQUESTED                                                                       Indication of remote OpenChannel                                              request.                                                        wParam        Channel handle                                                  lparam        Pointer to Channel Request information                                        structure:                                                                    {                                                                           DWORD      TransId (to be                                                                preserved in                                                                  Accept/                                                                       RejectChannel)                                                     HCONN      Connection handle                                                  LPCHAN.sub.-- INFO                                                                       Pointer to                                                                    CHAN.sub.-- INFO                                                              passed by                                                                     remote application                                                 }                                                                 CHAN.sub.-- ACCEPTED                                                                        Response to OpenChannel request.                                wParam        Channel handle                                                  lParam        TransID specified by application in                                           OpenChannel request                                             CHAN.sub.-- REJECTED                                                                        Response to OpenChannel request.                                lParam        TransID specified by application in                                           OpenChannel request                                             CHAN.sub.-- CLOSED                                                                          Indication of remote CloseChannel.                              wParam        Channel handle                                                  CHAN.sub.-- CLOSE.sub.-- RESP                                                               Response to CloseChannel request.                               wParam        Channel handle                                                  lParam        TransID specified by application in                                           CloseChannel                                                    ______________________________________                                    

Channel Handler Messages

The following messages are generated in response to the various channelI/0 functions as described with the function definitions:

    ______________________________________                                        CHAN.sub.-- DATA.sub.-- SENT                                                                  Response to SendData.                                         wParam          Actual bytes sent                                             lParam          TransID specified by application in                                           SendData                                                      CHAN.sub.-- RCV.sub.-- COMPLETE                                                               Response to ReceiveData.                                      wParam          Actual bytes received                                         lParam          TransID specified by application in                                           ReceiveData                                                   CHAN.sub.-- DATA.sub.-- LOST                                                  wParam          Bytes discarded                                               lParam          TransID specified by application                              ______________________________________                                    

Data Structures

The following are the important data structures for the comm subsystem:

    ______________________________________                                        TADDR, LPTADDR:    Address structure for                                                         caller/callee.                                             CHAN.sub.-- INFO, LPCHAN.sub.-- INFO:                                                            Channel information                                                           structure.                                                 CONN.sub.-- CHR, LPCONN.sub.-- CHR:                                                              Connection Attributes                                                         structure.                                                 ______________________________________                                    

The column subsystem provides two different methods of eventnotification to the conferencing applications: Microsoft® Windowsmessages and callbacks. A conferencing application program instructs thecomm subsystem as to which method should be used for notification ofdifferent events. Microsoft® Windows messages employ the Microsoft®Windows messaging mechanism to notify the conferencing application thatan event has occurred. For callbacks, the comm subsystem calls a userprocedure when an event has taken place. There are restrictions on whatthe conferencing application may or may not do within a callbackroutine.

Referring now to FIG. 19, there is shown a representation of the commsubsystem application finite state machine (FSM) for a conferencingsession between a local conferencing system (i.e., local site or caller)and a remote conferencing system (i.e., remote site or callee),according to a preferred embodiment of the present invention. Thepossible application states are as follows:

    ______________________________________                                        INIT      Initial or null state                                               IN.sub.-- SESSION                                                                       Conferencing session begun                                          CONN.sub.-- IN                                                                          Incoming connection request received                                          from remote site                                                    CONN.sub.-- OUT                                                                         Outgoing connection request made to remote site                     CONNCTED  Connection accepted (by local site for incoming                               connection and by remote site for outgoing                                    connection)                                                         CHAN.sub.-- IN                                                                          Incoming channel request received from remote                                 site                                                                CHAN.sub.-- OUT                                                                         Outgoing channel request made to remote site                        RECEIVE   Incoming channel accepted by local site                             SEND      Outgoing channel accepted by remote site                            ______________________________________                                    

Referring now to FIG. 20, there is shown a representation of the commsubsystem connection FSM for a conferencing session between a local siteand a remote site, according to a preferred embodiment of the presentinvention. The possible connection states are as follows:

    ______________________________________                                        NULL            Null state                                                    IDLE            Idle state                                                    AWAIT.sub.-- LOCAL.sub.-- RESP                                                                Awaiting response from local site                             AWAIT.sub.-- ACCEPT.sub.-- RESP                                                               Awaiting acceptance response                                  AWAIT.sub.-- REMOTE.sub.-- RESP                                                               Awaiting response from remote site                            ALIVE           Connection is alive                                           ESTABLISHED     Connection is established                                     ______________________________________                                    

Referring now to FIG. 21, there is shown a representation of the commsubsystem control channel handshake FSM for a conferencing sessionbetween a local site and a remote site, according to a preferredembodiment of the present invention. The possible control channelhandshake states are as follows:

    ______________________________________                                        NULL              Null state                                                  AWAIT.sub.-- CTL.sub.-- OPEN                                                                    Awaiting opening of control                                                   channel 0                                                   AWAIT.sub.-- ALIVE.sub.-- MESSAGE                                                               Awaiting message that control                                                 channel is alive                                            CTL.sub.-- ESTABLISHED                                                                          Control channel established                                 ______________________________________                                    

Referring now to FIG. 22, there is shown a representation of the commsubsystem channel establishment FSM for a conferencing session between alocal site and a remote site, according to a preferred embodiment of thepresent invention. The possible channel establishment states are asfollows:

    ______________________________________                                        NULL                Null state                                                IDLE                Idle state                                                CHAN.sub.-- AWAIT.sub.-- DLM.sub.-- OPN.sub.-- RX                                                 Awaiting DLM to open                                                          receive channel                                           AWAIT.sub.-- LOCAL.sub.-- RESP                                                                    Awaiting local application                                                    response to request to                                                        open receive channel                                      CHAN.sub.-- RECEIVING                                                                             Receive channel open                                      CHAN.sub.-- AWAIT.sub.-- DLM.sub.-- OPN.sub.-- TX                                                 Awaiting DLM to open                                                          send channel                                              AWAIT.sub.-- REM.sub.-- RESP                                                                      Awaiting remote appli-                                                        cation response to request                                                    to open send channel                                      CHAN.sub.-- SENDING Send channel open                                         ______________________________________                                    

Referring now to FIG. 23, there is shown a representation of the commsystem processing for a typical conferencing session between a callerand a callee, according to a preferred embodiment of the presentinvention. Both the caller and callee call the BeginSession function tobegin the conferencing session. The caller then calls the MakeConnectionfunction to initiate a connection to the callee, which causes aConnectRequest message to be sent to the callee. The callee responds bycalling the AcceptConnection function, which causes a ConnectAcceptmessage to be sent to the caller and the callee.

Both the caller and callee then call the RegisterChanMan function toregister the channel. Both the caller and callee then call theOpenChannel function to open a channel to the other, which causesChannelRequest messages to be exchanged between the caller and callee.Both the caller and callee call the AcceptChannel function to accept thechannel requested by the other, which causes ChannelAccepted messages tobe exchanged between the caller and callee. Both the caller and calleecall the RegisterChanHandler function two times to register both theincoming and outgoing channels.

The callee calls the ReceiveData function to be ready to receive datafrom the caller. The caller then calls the SendData function, whichcauses conferencing data to be sent to the callee. The caller receives alocally generated DataSent message with the sending of the data iscomplete. The callee receives a ReceiveComplete message when the receiptof the data is complete. Note that the caller does not receive a messageback from the callee that the data was successfully received by thecallee.

The scenario of FIG. 23 is just one possible scenario. Those skilled inthe an will understand that other scenarios may be constructed usingother function calls and state transitions.

Comm Manager

The comm manager 518 of FIG. 5 comprises three dynamically linkedlibraries of FIG. 17: transport independent interface (TII), reliabledatalink module (RDLM.DLL) and datalink module interface (DLM.DLL). TheDLM interface is used by the TII to access the services of the ISDNaudio/comm board 206. Other modules (i.e., KPDAPI.DLL and DSP.DRV)function as the interface to the audio/comm board and have no otherfunction (i.e., they provide means of communication between the hostprocessor portion of the DLM and the audio/comm portion of the DLM. Thehost processor portion of the DLM (i.e., DLM.DLL) uses the DSP interface528 of FIG. 5 (under Microsoft® Windows 3.x) to communicate with theISDN audio/comm board side portions. The DLM interface and functionalitymust adhere to the DLM specification document.

The TII provides the ability to specify whether or not a virtual channelis reliable. For reliable channels, TII employs the RDLM to providereliability on a virtual channel. This feature is used to indicate thatthe audio and video virtual channels are unreliable, and the datavirtual channel is reliable.

Data Link Manager

The DLM subsystem maintains multiple channels between the clients andsupports data transfers up to 64K per user message. The upper layerusing DLM assumes that message boundaries are preserved (i.e., userpackets are not merged or fragmented when delivered to the upper layerat the remote end).

Before data can be transferred via DLM, the two communicating machineseach establish sessions and a connection is set up between them. Thissection details the functions used to establish sessions andconnections. DLM provides the following functions for call control:

DLM₋₋ BeginSession

DLM₋₋ EndSession

DLM₋₋ Listen

DLM₋₋ MakeConnection

DLM₋₋ AcceptConnection

DLM₋₋ RejectConnection

DLM₋₋ CloseConnection

The following calls should be allowed in an interrupt context: DLM₋₋MakeConnection, DLM₋₋ AcceptConnection, DLM₋₋ RejectConnection, andDLM₋₋ CloseConnection. These functions may generate the followingcallbacks to the session callback handler, described below.

CONN₋₋ REQUESTED

CONN₋₋ ESTABLISHED

CONN₋₋ REJECTED

CONN₋₋ CLOSE COMPLETE

CONN₋₋ CLOSE NOTIFY

SESS₋₋ CLOSED

SESS₋₋ ERROR

CONN₋₋ ERROR

Most of the session and connection management functions of the DLM₋₋ areasynchronous. They initiate an action and when that action is complete,DLM₋₋ will call back to the user via the session callback. The callingconvention for the callback is as follows:

    ______________________________________                                        void FAR PASCAL ConnectionCallback (LPEVENTSTRUCT                             Event);                                                                       Event is a far pointer to a structure:                                        struct EVENTSTRUCT                                                            WORD            EventType;                                                    WORD            Status;                                                       BYTE            DlmId;                                                        BYTE            MdmId;                                                        DWORD           DlmSessionId;                                                 DWORD           DlmConnId;                                                    DWORD           Token;                                                        LPTADDR         Addr;                                                         LPCONNCHR       Characteristics;                                              }                                                                             where:                                                                        EventType Specifies the type of event which triggered the                               callback.                                                           Status    Indicates the status of the event.                                  DlmId     Unique ID of the DLM performing the callback.                                 (Equals 0 for DGM&S.)                                               MdmId     Unique ID of the MDM that processed the                                       event. (Equals 0 for DGM&S.)                                        DlmSessionId                                                                            Indicates the Session ID, assigned by DLM, on                                 which this event occurred. (Equals 0 for                                      DGM&S.)                                                             DlmConnId Indicates the Connection Id, assigned by DLM,                                 on which this event occurred. (Equals 0 for                                   DGM&S.)                                                             Token     The token value was given in the call to initiate                             an action. When the callback notifies the user                                that the action is complete, the token is returned                            in this field.                                                      Addr      Specifies the LPTADDR of the caller.                                Characteristics                                                                         This field is a LPCONNCHR to the connection                                   characteristics.                                                    ______________________________________                                    

For each function defined below which generates a callback, all of thefields of the DLM₋₋ event structure are listed. If a particular fieldcontains a valid value during a callback, an X is placed in the tablefor the callback. Some fields are only optionally returned by the DLM₋₋(and underlying MDMs). Optional fields are noted with an `O` in thetables. If a pointer field is not valid or optionally not returned theDLM₋₋ will pass a NULL pointer in its place. The upper layer should notassume that pointer parameters such as LPEVENTSTRUCT, LPTADDR, andLPCONNCHR are in static memory. If the upper layer needs to process themin a context other than the callback context it should make a privatecopy of the data. ##SPC1##

Referring now to FIG. 29, there are shown diagrams indicating typicalconnection setup and teardown sequences.

Interfaces--Channel Management & Data Transfer

Once connections are established between two machines, DLM₋₋ willprovide the user with multiple logical channels on the connections. Thissection details the functions and callbacks used to set up, tear down,and send data on channels. DLM₋₋ has the following entry points forchannel management and data transfer.

DLM₋₋ Open

DLM₋₋ Send

DLM₋₋ PostBuffer

DLM₋₋ Close

DLM₋₋ GetCharacteristics

Each of these functions is callable from an interrupt or callbackcontext. These functions generate callbacks into the user's code forcompletion of a send operation, receipt of data, and events occurring ona given channel. These callbacks are described and their profiles givena later section of this specification.

    ______________________________________                                        DLM.sub.-- Open                                                                        Initializes a new data channel for a connection.                              It does not communicate with the remote site.                                 Its role is simply to declare the channel identifier                          to the DLM so that incoming and outgoing packets                              can then use the given channel.                                      WORD DLM.sub.-- Open                                                                       (DWORD ConnID,                                                                BYTE ChannelID,                                                               LPCHANCHR Characteristics,                                                    FARPROC EventCallback,                                                        FARPROC ReceiveCallback,                                                      FARPROC SendCallback)                                            Parameters:                                                                   ConnID:      Connection on which to open the channel.                         ChannelID    Identifier of the channel to open, between                                    0 and N where N is implementation                                             defined. The value of 255 is reserved to                                      indicate an unknown or invalid channel in                                     callback functions.                                              Characteristics                                                                            Desired characteristics of the channel.                          EventCallback                                                                              Callback function for events occurring on                                     this channel. (This includes all events                                       except for data received and send                                             complete)                                                        ReceiveCallback                                                                            Callback function for data reception on                                       this channel.                                                    SendCallback Callback function for data sent on this                                       channel.                                                         Return Value:                                                                              Status Indication                                                E.sub.-- NOCHAN                                                                            Unable to allocate channel ID or ID                                           already in use.                                                  E.sub.-- SESSNUM                                                                           ConnID is not valid.                                             E.sub.-- SESSUNUSED                                                                        Session is not in use.                                           E.sub.-- SESSCLOSED                                                                        Session has been closed.                                         E.sub.-- SESSNOTOPEN                                                                       Session is not open.                                             E.sub.-- IDERR                                                                             ConnID does not refer to a connection on                                      this DLM.                                                        E.sub.-- CONNNUM                                                                           ConnID is not valid.                                             E.sub.-- CONNUNUSED                                                                        Connection is not in use.                                        E.sub.-- CONNCLOSED                                                                        Connection has been closed.                                      E.sub.-- CONNNOTOPEN                                                                       Connection is not currently open.                                Local Callbacks:                                                              CHANNELOPEN callback to the event callback for this                           channel.                                                                      DLM.sub.-- Send                                                                        Entry point for sending data via the DLM.                            WORD DLM.sub.-- Send                                                                       (DWORD ConnID,                                                                BYTE FAR *Buffer,                                                             WORD BufferSize,                                                              BYTE OriginatingChannel,                                                      BYTE ReceivingChannel,                                                        DWORD CallerToken)                                               Parameters:                                                                   ConnID:      Connection to use.                                               Buffer       Far pointer to the user buffer to send.                          BufferSize   Number of bytes in the user buffer.                              OriginatingChannel                                                                         Local channel on which to send the data.                         ReceivingChannel                                                                           Channel ID from the remote machine                                            which receives the data.                                         CallerToken  Token which will be returned to the user                                      in the send complete callback for this                                        buffer.                                                          Return Value:                                                                              Status Indication                                                E.sub.-- NOCHAN                                                                            Originating channel is not valid or is                                        closed.                                                          E.sub.-- SESSNUM                                                                           ConnID is not valid.                                             E.sub.-- SESSUNUSED                                                                        Session is not in use.                                           E.sub.-- SESSCLOSED                                                                        Session has been closed.                                         E.sub.-- SESSNOTOPEN                                                                       Session is not open.                                             E.sub.-- IDERR                                                                             ConnID does not refer to a connection on                                      this DLM.                                                        E.sub.-- CONNNUM                                                                           ConnID is not valid.                                             E.sub.-- CONNUNUSED                                                                        Connection is not in use.                                        E.sub.-- CONNCLOSED                                                                        Connection has been closed.                                      E.sub.-- CONNNOTOPEN                                                                       Connection is not currently open.                                E.sub.-- CHANNUM                                                                           Originating channel ID is not valid.                             E.sub.-- CHANUNUSED                                                                        Originating channel is not in use.                               E.sub.-- CHANCLOSED                                                                        Originating channel is closed.                                   E.sub.-- NOMEM                                                                             Unable to allocate enough memory to                                           perform the send.                                                E.sub.-- INTERNAL                                                                          An internal error has occurred within the                                     DLM.                                                             Local Callbacks:                                                              Callback to the send complete function for this channel when this             buffer is posted to the net.                                                  ______________________________________                                    

The return value of DLM₋₋ Send specifies the synchronous status of thesend. If it indicates success, the request has been accepted to be senton the network for this channel and at some time the send completecallback will be activated for this buffer. Between the call to DLM₋₋Send and the send complete callback, the user must not change thecontents of the buffer. When the callback occurs, DLM is finished withthe buffer and the user is free to alter it in any fashion. The DLM doesnot guarantee that the call to DLM₋₋ Send completes before the sendcomplete callback occurs. If the synchronous status indicates that thesend operation has failed, the send complete callback will not beactivated for this buffer and the buffer is immediately available formodification by the user.

    ______________________________________                                        DLM.sub.-- PostBuffer                                                                      Supplies buffers to DLM in which to                                           place incoming data.                                             WORD DLM.sub.-- PostBuffer                                                                   (DWORD ConnID,                                                                BYTE FAR *Buffer,                                                             WORD BufferSize,                                                              BYTE ChannelID,                                                               DWORD CallerToken)                                             Parameters:                                                                   ConnID:      Connection to use.                                               Buffer       Far pointer to the user buffer to use.                           BufferSize   Size of the user buffer in bytes.                                ChannelID    Local channel to use this buffer for.                            CallerToken  Token which will be returned to the user                                      in the data receive callback for this                                         buffer.                                                          Return Value:                                                                              Status Indication                                                E.sub.-- NOCHAN                                                                            ChannelID is not valid or is closed.                             E.sub.-- SESSNUM                                                                           ConnID is not valid.                                             E.sub.-- SESSUNUSED                                                                        Session is not in use.                                           E.sub.-- SESSCLOSED                                                                        Session has been closed.                                         E.sub.-- SESSNOTOPEN                                                                       Session is not open.                                             E.sub.-- IDERR                                                                             ConnID does not refer to a connection on                                      this DLM.                                                        E.sub.-- CONNNUM                                                                           ConnID is not valid.                                             E.sub.-- CONNUNUSED                                                                        Connection is not in use.                                        E.sub.-- CONNCLOSED                                                                        Connection has been closed.                                      E.sub.-- CONNNOTOPEN                                                                       Connection is not currently open.                                E.sub.-- CHANNUM                                                                           ChannelID is not valid.                                          E.sub.-- CHANUNUSED                                                                        Channel is not in use.                                           E.sub.-- CHANCLOSED                                                                        Channel is closed.                                               E.sub.-- NOMEM                                                                             Unable to allocate enough memory to                                           store the buffer.                                                E.sub.-- INTERNAL                                                                          An internal error has occurred within                                         the DLM.                                                         Local Callbacks:                                                              Callback to the data receive function for this channel when DLM               loads the user buffer with incoming data.                                     ______________________________________                                    

The return value is a word indicating the status of the operation. If itindicates success, the buffer has been enqueued for the given channeland will be used for incoming data. If it indicates failure, a receivecallback will never occur for this buffer. DLM preserves the order ofbuffers on data receives. Provided that no errors occur, the firstbuffer posted will be the first one used for data, the second one willbe the second used, etc.

    ______________________________________                                        DLM.sub.-- Close                                                                           Used to close a previously opened                                             channel.                                                         WORD DLM.sub.-- Close                                                                      (WORD ConnID,                                                                 BYTE Channel)                                                    Parameters:                                                                   ConnID:      Connection on which to close the channel.                        Channel      Local channel to close.                                          Return Value:                                                                              Status Indication                                                E.sub.-- SESSNUM                                                                           ConnID is not valid.                                             E.sub.-- SESSUNUSED                                                                        Session is not in use.                                           E.sub.-- SESSCLOSED                                                                        Session has been closed.                                         E.sub.-- SESSNOTOPEN                                                                       Session is not open.                                             E.sub.-- IDERR                                                                             ConnID does not refer to a connection on                                      this DLM.                                                        E.sub.-- CONNNUM                                                                           ConnID is not valid.                                             E.sub.-- CONNUNUSED                                                                        Connection is not in use.                                        E.sub.-- CONNCLOSED                                                                        Connection has been closed.                                      E.sub.-- CONNNOTOPEN                                                                       Connection is not currently open.                                E.sub.-- CHANNUM                                                                           Channel is not valid.                                            E.sub.-- CHANUNUSED                                                                        Channel is not in use.                                           E.sub.-- CHANCLOSED                                                                        Channel is already closed.                                       Local Callbacks:                                                              Callback to the event callback function for this channel with the             CHANNELCLOSED event after the close has completed.                            ______________________________________                                    

The function DLM₋₋ Close shuts down a given channel. All futurereferences to this channel are considered invalid. It performs a forcedshutdown in that the callback functions for all pending sends andreceives are immediately activated with a status value indicating that aclose occurred. DLM does not guarantee that the call to DLM₋₋ Close willreturn before the callback is activated.

    ______________________________________                                        DLM.sub.-- GetCharacteristics                                                                Gets relevant data about the DLM (a                                           synchronous call).                                             WORD DLM.sub.-- GetCharacteristics(LPCHARSTRUCT                               Characteristics)                                                              Parameters:                                                                   LPCHARSTRUCT                                                                              Far pointer to the characteristics structure                                  to be filled by this call.                                        Local Callbacks:                                                              None                                                                          ______________________________________                                    

Send Callback

The send complete callback is activated whenever data has been extractedfrom a user's buffer and enqueued for transmission. It is not aguarantee that the data has actually been delivered to the remote site.The entry point for the send complete callback is defined SendCallbackparameter to DLM₋₋ Open. This is a far pointer to a far pascal functiondefined as follows.

    ______________________________________                                        void FAR PASCAL SendCallback                                                                     (DWORD ConnID,                                                                BYTE FAR *BufferSent,                                                         WORD ByteCount,                                                               BYTE OriginatingChannel,                                                      BYTE ReceivingChannel,                                                        DWORD Token,                                                                  WORD StatusOfSend)                                         Parameters:                                                                   ConnID:     Connection on which data was sent.                                Buffer      Far pointer to the user buffer sent.                              BufferSize  Number of bytes sent to the network.                              OriginatingChannel                                                                        Local channel on which to the data was                                        sent.                                                             ReceivingChannel                                                                          Channel ID from the remote machine which                                      will receive the data.                                            CallerToken Token which was given in the call to                                          DLM.sub.-- Send for this buffer.                                  ______________________________________                                    

Data Receive Callback

The data receive callback is activated when data has arrived on thenetwork for a particular channel. The entry point for the data receivecallback is defined in the ReceiveCallback parameter to DLM₋₋ Open,described below. It must be a far pointer to a far pascal functiondefined as follows:

    __________________________________________________________________________    void FAR PASCAL ReceiveCallback                                                                  (DWORD ConnID,                                                                BYTE FAR *BufferReceived,                                                     WORD ByteCount,                                                               BYTE OriginatingChannel,                                                      BYTE ReceivingChannel                                                         DWORD Token,                                                                  WORD StatusOfReceive)                                      Parameters:                                                                   ConnID:    Connection on which the data was received.                         BufferReceived                                                                           The user supplied buffer that was received.                        ByteCount  The number of bytes received.                                      OriginatingChannel                                                                       Channel identifier of the channel on the remote machine which                 sent                                                                          the data.                                                          ReceivingChannel                                                                         Channel identifier on the local machine that received the                     data.                                                              Token      Token value that was given in DLM.sub.-- PostBuffer when this                 buffer was                                                                    posted to DLM.                                                     StatusOfReceive                                                                          Status of the operation.                                           The StatusOfReceive parameter can be any of the following values:             E.sub.-- OK                                                                              Indicates that the receive succeeded.                              E.sub.-- TOOSMALL                                                                        Indicates that the beginning of a data packet has arrived and                 the given                                                                     buffer was enqueued but it is too small to contain the entire                 data packet.                                                       E.sub.-- CLOSED                                                                          Indicates that the buffer was in the receive queue when the                   channel on                                                                    the local machine was closed.                                      E.sub.-- DATADROP                                                                        Indicates that a data packet has arrived and there is no                      buffer in the                                                                 queue for the receiving channel.                                   E.sub.-- PARTIAL                                                                         Indicates that part of a data packet has been dropped, either                 by the                                                                        network or by internal memory limitations of the MDM or DLM.                  The                                                                           buffer represents everything received up to the dropped            __________________________________________________________________________               data.                                                          

The state of the parameters depends on the status of the operation. Thetable below lists all possible status values correlating them with thevalues returned in the other parameters, and entry of Valid indicatesthat this parameter contains meaningful data. The connection ID isalways valid.

    __________________________________________________________________________                          Original                                                                            Receiving                                         Status    Buffer                                                                             ByteCount                                                                            Channel                                                                             Channel                                                                              Token                                      __________________________________________________________________________    E.sub.-- OK                                                                             Valid                                                                              Valid  Valid Valid  Valid                                      E.sub.-- TOOSMALL                                                                       Valid             Valid  Valid                                      E.sub.-- CLOSED                                                                         Valid             Valid  Valid                                      E.sub.-- DATADROP                                                                       NULL        Valid Valid                                             E.sub.-- PARTIAL                                                                        Valid                                                                              Valid  Valid Valid  Valid                                      __________________________________________________________________________

When errors E₋₋ TOOSMALL, E₋₋ DATADROP or E₋₋ PARTIAL are returned theupper layer may not depend on the contents of the returned data buffer.

    ______________________________________                                        EventCallback                                                                           Activated when an action completes for a given                                channel. The entry point for the channel event                                callback is defined in the EventCallback                                      parameter to DLM.sub.-- Open. It is a far pointer to a                        far pascal function defined as follows.                             void FAR PASCAL EventCallback                                                                     (DWORD ConnID,                                                                BYTE Channel,                                                                 WORD Event,                                                                   WORD Status)                                              Parameters:                                                                   ConnID: Connection on which the event occurred.                               Channel Channel on which the event occurred.                                  Event   The type of the event                                                 Status  Status of the operation.                                              The event may be any of the following values.                                 CHANNEL.sub.-- OPEN                                                                         The given channel has been opened and                                         is now available for data transfer.                             CHANNEL.sub.-- CLOSED                                                                       The given channel has been closed.                              ______________________________________                                    

DSP Interface

The ISDN comm task 540 of FIG. 5 which run on the ISDN audio/comm board206 of FIG. 2 communicate with the host processor 202 via the DSPinterface 528. The host processor operates under Microsoft® Windows 3.xenvironment.

Comm Task

The comm task 540 of FIG. 5 communicates with the audio task 538 on theISDN audio/comm board 206. The channel ID of the audio virtual channelis accessible to both the host processor and the audio/comm board. Themodel is as follows:

A channel is opened by the host processor or an open channel request isgranted by the host processor.

The host processor signals the audio task on the audio/comm board that achannel is accepted/opened on its behalf.

The audio task on the audio/comm board notifies the comm task that allincoming (if the channel was accepted) or outgoing (if the channel wasopened) will be handled by the on-board audio task.

Application-Level Protocols

The application-level protocols for conferencing system 100 of FIG. 5are divided into those for the video, audio, and data streams.

Video Protocol

Referring now to FIG. 24, there is shown a representation of thestructure of a video packet as sent to or received from the commsubsystem, according to a preferred embodiment of the present invention.Source video is video that is captured (and optionally monitored) on thelocal conferencing system and sent to the comm subsystem fortransmission to a remote system. Sink video is video that is capturedremotely, received from the comm subsystem, and played back on the localsystem. The first ten fields (i.e., those from lpData throughdwReserved[3]) are defined by Microsoft® as the VIDEOHDR structure. Seethe Microsoft® Programmer's Guide in the Microsoft® Video for WindowsDevelopment Kit. The video packet fields are defined as follows:

    ______________________________________                                        lpData     Long pointer to the video frame data buffer.                       dwBufferLength                                                                           Length of the data buffer pointed to by                                       lpData, in bytes.                                                  dwBytesUsed                                                                              Length of bytes used in the data buffer.                           dwTimeCaptured                                                                           Time, in milliseconds, between the current                                    frame and the beginning of the capture session.                               This field is preferably used to carry a                                      timestamp used to synchronize audio and video                                 frames at the receiving endpoint.                                  dwUser     Reserved for application use.                                      dwFlags    Information about the data buffer, defined                                    flags are:                                                                  VHDR.sub.-- DONE                                                                            Data buffer is ready                                                          for the application.                                            VHDR.sub.-- INQUEUE                                                                         Data buffer is                                                                queued pending                                                                playback.                                                       VHDR.sub.-- KEYFRAME                                                                        Data buffer is a key                                                          frame.                                                          VHDR.sub.-- PREPARED                                                                        Data buffer has                                                               been prepared for                                                             use by the driver.                                     dwReserved Reserved for driver use.                                           Type       Type of the packet, defined types are:                                      VDATA (=1)                                                                              Video data packet.                                                  VCNTL (=2)                                                                              Control packet.                                            Message    Unused for video data packets. For control                                    packets, may be one of the following:                                         RESTART (=WM.sub.-- USER+550h) Request                                        for a key frame.                                                              When a RESTART control packet is sent,                                        no video frame data is sent. WM.sub.-- USER is a                              Microsoft ® Windows defined value and is                                  preferably 400h. RESTART indicates the                                        video stream needs to be restarted to recover                                 from problems. WM.sub.-- USER is a                                 defined constant, indicating                                                             that all values greater than this number                                      are application-defined constants.                                 Data       Compressed video frame data.                                       ______________________________________                                    

Video data packets are used to exchange actual video frame data and areidentified by the Type field. In this case, the video software redirectsthe VIDEOHDR lpData pointer to the Data array which starts at the end ofthe packet. In this way, the packet header and data are kept contiguousin linear memory. The VIDEOHDR dwBufferLength field is used to indicatethe actual amount of video data in the buffer and therefore the amountof data to be sent/received. Note that the receiving application mustredirect lpData to its copy of Data since the memory pointer only haslocal significance. In a preferred embodiment, Data length has an upperbound of 18K bytes.

Compressed Video Bitstream

Referring now to FIG. 25, there is shown a representation of thecompressed video bitstream for conferencing system 100, according to apreferred embodiment of the present invention. Each compressed videobitstream represents one frame of video data stored in the Data fieldfor a video data packet of FIG. 24. The video compression/decompressionmethod associated with the compressed video bitstream of FIG. 25 is usedfor low-data-rate, relatively-low-frame-rate, teleconferencingapplications. The method preferably operates at approximately (160×120)resolution, a data rate of approximately 100 Kb/sec, and a frame rate ofaround 10 frames/sec. Under these conditions, the compressed videobitstream may be encoded or decoded in real-time by an Intel® i750®processor, or decoded in real-time by an Intel® architecture processorsuch as an Intel® 80386, 80486, or Pentium® processor.

The fields of the compressed video bitstream of FIG. 25 are defined asfollows:

    ______________________________________                                        VersionNumber                                                                            Compression method ID.                                             Flags      Contains various flag bits defined as follows:                              FLAGS.sub.-- MV                                                                              1                                                              FLAGS.sub.-- FILTER                                                                          2                                                              FLAGS.sub.-- STILL.sub.-- IMAGE                                                              4                                                              FLAGS.sub.-- STILL.sub.-- BLKS                                                               8                                                     DataSize   Size of the bitstream in units of bits.                            Reservedl  Reserved field.                                                    ImageHeight                                                                              Height of image in pixels.                                         ImageWidth Width of image in pixels.                                          UVquant    Base quantization value for the U and V                                       planes.                                                            Yquant     Base quantization value for the Y plane.                           StillStrip Strip of blocks encoded as still blocks (for                                  delta images only). If StillStrip = 0, there                                  is no still strip. Otherwise, the strip of                                    blocks is determined as follows. Consider the                                 blocks of the Y, V, and U planes in raster                                    order as a linear sequence of blocks. Divide                                  this sequence of blocks into groups of 4 blocks,                              and number each group with the sequential                                     integers 1, 2, 3, etc. These numbers correspond                               to the value of StillStrip. In a preferred                                    embodiment, all planes have dimensions                                        that are integer multiples of 4.                                   StillThresh                                                                              Locations of additional blocks in the image                                   that are encoded as still blocks (only if the                                 FLAGS.sub.-- STILL.sub.-- BLKS flag is set). The                              rule for identifying these blocks is based on                                 the quantization value quant for each block as                                determined during the decoding procedure.                                     A block is a still block if                                                      quant <= StillThresh                                                       These still blocks are independent of the                                     blocks in the still strip, which are encoded                                  as still blocks regardless of their quant values.                  FilterThresh                                                                             Blocks to which the loop filter is to be applied                              (only if the FLAGS.sub.-- FILTER flag is set) The                             rule for applying the loop filter is to apply it                              to a block if                                                                    quant <= FilterThresh                                           MotionVectors[]                                                                          Array describing the motion vectors used                                      in decoding the image (only present if the                                    FLAGS.sub.-- MV flag is set). There is one 8-bit                              motion vector field for each (16 × l6) block                            in the image.                                                      huffman data                                                                             The compressed data for the image.                                 ______________________________________                                    

FLAGS₋₋ MV indicates whether motion vectors are present in the bitstream(i.e., whether the MotionVectors[]array is present). A delta frame withFLAGS₋₋ MV=0 is interpreted as one in which all the motion vectors are0. FLAGS₋₋ FILTER indicates whether the loop filter is enabled for thisimage. If enabled, then the loop filter may be used on each block in theimage, as determined by the value of FilterThresh. FLAGS₋₋ STILL₋₋ IMAGEindicates whether the image is a still frame or a delta (non-still)frame. A still frame is one in which all blocks are encoded as stillblocks. In a delta frame, most blocks are delta blocks, but there may bea strip of still blocks in the image, as specified by the StillStripfield, and there may be additional still blocks as determined by thevalue of StillThresh. FLAGS₋₋ STILL₋₋ BLKS indicates whether "additionalstill blocks" are enabled for this image. If enabled, then any blockwith quantization value less than or equal to StillThresh is coded as astill block.

A quantization value is a number in the range 0-15 that indicates one ofa set of sixteen (8×8) quantization matrices, with 0 indicating thecoarsest quantization and 15 indicating the finest. The UVquant andYquant variables are referred to as base quantization values. The basequantization value is the value selected for use at the beginning of aplane, and is used for the entire plane unless changed by a NEWQ codeinserted in the bitstream. The preferred 16 quantization matrices are:

    ______________________________________                                        5 5 6 6 7 7 8 8                                                               5 5 6 6 7 7 8 8                                                               6 6 6 6 7 7 8 8                                                               6 6 6 6 7 7 8 8                                                               7 7 7 7 7 7 8 8                                                               7 7 7 7 7 7 8 8                                                               8 8 8 8 8 8 8 8                                                               8 8 8 8 8 8 8 8                                                               5 4 5 5 6 6 7 7                                                               4 4 5 5 6 6 7 7                                                               5 5 5 5 6 6 7 7                                                               5 5 5 5 6 6 7 7                                                               6 6 6 6 6 6 7 7                                                               6 6 6 6 6 6 7 7                                                               7 7 7 7 7 7 7 7                                                               7 7 7 7 7 7 7 7                                                               5 4 5 5 5 5 7 7                                                               4 4 5 5 5 5 7 7                                                               5 5 5 5 5 5 7 7                                                               5 5 5 5 5 5 7 7                                                               5 5 5 5 5 5 7 7                                                               5 5 5 5 5 5 7 7                                                               7 7 7 7 7 7 7 7                                                               7 7 7 7 7 7 7 7                                                               5 4 5 5 5 5 6 6                                                               4 4 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               6 6 6 6 6 6 6 6                                                               6 6 6 6 6 6 6 6                                                               5 4 5 5 5 5 6 6                                                               4 4 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               6 6 6 6 6 6 6 6                                                               6 6 6 6 6 6 6 6                                                               5 4 4 4 5 5 6 6                                                               4 4 4 4 5 5 6 6                                                               4 4 4 4 5 5 6 6                                                               4 4 4 4 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               6 6 6 6 6 6 6 6                                                               6 6 6 6 6 6 6 6                                                               5 4 4 4 5 5 5 5                                                               4 4 4 4 5 5 5 5                                                               4 4 4 4 5 5 5 5                                                               4 4 4 4 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               4 3 4 4 5 5 6 6                                                               3 3 4 4 5 5 6 6                                                               4 4 4 4 5 5 6 6                                                               4 4 4 4 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               5 5 5 5 5 5 6 6                                                               6 6 6 6 6 6 6 6                                                               6 6 6 6 6 6 6 6                                                               4 3 4 4 5 5 5 5                                                               3 3 4 4 5 5 5 5                                                               4 4 4 4 5 5 5 5                                                               4 4 4 4 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               4 3 4 4 4 4 5 5                                                               3 3 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               4 3 3 3 4 4 5 5                                                               3 3 3 3 4 4 5 5                                                               3 3 3 3 4 4 5 5                                                               3 3 3 3 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               4 4 4 4 4 4 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               4 3 3 3 4 4 4 4                                                               3 3 3 3 4 4 4 4                                                               3 3 3 3 4 4 4 4                                                               3 3 3 3 4 4 4 4                                                               4 4 4 4 4 4 4 4                                                               4 4 4 4 4 4 4 4                                                               4 4 4 4 4 4 4 4                                                               4 4 4 4 4 4 4 4                                                               4 3 3 3 3 3 5 5                                                               3 3 3 3 3 3 5 5                                                               3 3 3 3 3 3 5 5                                                               3 3 3 3 3 3 5 5                                                               3 3 3 3 3 3 5 5                                                               3 3 3 3 3 3 5 5                                                               5 5 5 5 5 5 5 5                                                               5 5 5 5 5 5 5 5                                                               4 3 3 3 3 3 4 4                                                               3 3 3 3 3 3 4 4                                                               3 3 3 3 3 3 4 4                                                               3 3 3 3 3 3 4 4                                                               3 3 3 3 3 3 4 4                                                               3 3 3 3 3 3 4 4                                                               4 4 4 4 4 4 4 4                                                               4 4 4 4 4 4 4 4                                                               3 3 2 2 2 2 3 3                                                               3 3 2 2 2 2 3 3                                                               2 2 2 2 2 2 3 3                                                               2 2 2 2 2 2 3 3                                                               2 2 2 2 2 2 3 3                                                               2 2 2 2 2 2 3 3                                                               3 3 3 3 3 3 3 3                                                               3 3 3 3 3 3 3 3                                                               ______________________________________                                    

There is one motion vector per (16×16) block of the Y plane, listed inblock raster-scan order. The number of (16×16) blocks in the image, andhence the size of this array, can be determined from ImageHeight andImageWidth as:

    ((ImageHeight+15)>>4)*((ImageWidth+15)>>4)

In each byte of the MotionVector[] array, the upper 4 bits specifies theX component of the motion vector and the lower 4 bits specifies the Ycomponent (both in two's-complement notation). Both components of themotion vector are between +7 and -7, inclusive. The motion vectorspreferably apply to the Y plane only; the U and V planes are processedby the decoder using motion vectors of 0.

Video Decoding Procedure

For conferencing system 100, images are encoded in a 9-bit YUV format(i.e., YUV 4:1:1 format), in which there are three 8-bit planes ofpixels (Y, U, and V) with U and V subsampled by 4× in both directions.Each plane is subdivided into a grid of (8×8) blocks of pixels, and eachblock is encoded using a frequency-domain transform. The planes areencoded in the order Y, V, and U, and within each plane the blocks aretraversed in raster-scan order.

If a given plane's dimensions are not evenly divisible by 8, "partialblocks" at the right or bottom edges will occur. Partial blocks areencoded by padding them out to the full (8×8) size (using whatevermethod the encoder chooses, such as replicating the last column and/orrow or pixels) and encoding them as if they were full blocks. In thedecoder, such blocks are reconstructed by first decoding the full (8×8)block but then writing only the partial block to the final image bitmapin memory. The decoder can determine the location and sizes of partialblocks entirely from its knowledge of the image dimensions (ImageHeightand ImageWidth).

Each (8×8) block is encoded using a transform method. Instead of thediscrete cosine transform (DCT), a simpler transform known as thediscrete slant transform (DST) is used. The DST is almost as good at theDCT, in terms of compression and quality, but is simpler and faster forboth an Intel® i750® processor and an Intel® architecture processor suchas an Intel® 80386, 80486, or Pentium® processor to compute.

All the data in the bitstream, after the header, is Huffman encoded.Unlike H.261 and MPEG, which have a multiplicity of Huffman tables, forconferencing system 100, a single Huffman table is used for encoding allvalues. This single Huffman table is:

    ______________________________________                                               # codes                                                                ______________________________________                                                     0xx 4                                                                        10xxx                                                                              8                                                                       110xxxx                                                                             16                                                                     1110xxxxx                                                                            32                                                                    11110xxxxxx                                                                           64                                                                   111110xxxxxx                                                                           64                                                                  1111110xxxxxx                                                                           64                                                                  Total     252                                                          ______________________________________                                    

This table defines 252 Huffman codes of lengths 3, 5, 7, 9, 11, 12, and13 bits. Only the first 231 of these Huffman codes are preferably used;the remaining ones are reserved for future expansion.

In the pseudo-code below, the function huffdec() appears. This functiondoes a huffman-decoding operation on the next bits in the bitstream, andreturns the index of the code word in a lexicographically-ordered list,like so:

    ______________________________________                                        Code word    Value returned                                                   ______________________________________                                        000          0                                                                001          1                                                                010          2                                                                011          3                                                                10000        4                                                                10001        5                                                                10010        6                                                                etc.                                                                          ______________________________________                                    

The first step in decoding a block is to decode what are known as the"run/value pairs" (or run/val pairs, for short) for the block. Eachrun/val pair represents one non-zero DST frequency-domain coefficient.

This procedure also updates the current quantization value (held in thevariable quant) when a NEWQ code is received from the bitstream. Thevalue of quant is initialized at the start of each plane (Y, U, and V)to either Yquant or UVquant, but may be adjusted up or down by NEWQcodes in the bitstream. Note the following important rule, not madeexplicit by the pseudo-code below: a NEWQ code may preferably only occurat the beginning of a block. A decoder may use this fact to makedecoding faster, since it need not check for NEWQ codes in the middle ofparsing a block.

The procedure for decoding the run/val pairs and NEWQ codes is asfollows:

    __________________________________________________________________________    k = 0;                                                                        while (1)                                                                     v = huffdec ( );                                                              if (v == EOB)                                                                   break;                                                                      else if (v == NEWQ)                                                             quant += tosigned(huffdec( ));                                              else if (v == ESC)                                                                            //   get explicit run,val from                                                //   bitstream                                                {                                                                               run[k++] = huffdec( ) + 1;                                                    val[k++] = tosigned(huffdec( ) | (huffdec( ) << 6));               }                                                                             else            //   lookup run,val in tables                                 {                                                                               run[k++] = runtbl[v];                                                         val[k++] = valtbl[v];                                                       }                                                                             }                                                                             __________________________________________________________________________

The function tosigned() converts from an unsigned number to a non-zerosigned number, as follows:

    ______________________________________                                                 tosigned(n)                                                                   {                                                                                v = (n >> 1) + 1;                                                             if (n & 1) v = -v;                                                            return(v);                                                                 }                                                                    ______________________________________                                    

This conversion is used on both the quantization change and the explicitvalue read after an ESC, both of which are non-zero signed numbers. EOB,ESC, and NEWQ are specific decoded values defined as follows:

EOB=0

ESC=30

NEWQ=6

Finally, runtbl[] and valtbl[] are preferably defined as follows:

    __________________________________________________________________________    runtbl[ ] = {                                                                         0   1   1   2  2   1   0   1                                                  1   1   3   3  2   1   1   5                                                  4   4   5   6  6   3   1   2                                                  1   3   1   2  7   1   0   2                                                  7   9   8   4  1   5   1   1                                                  2   4   2   8  10  3   13  1                                                  1   1   1   1  1   11  2   15                                                 1   4   1   7  9   14  7   21                                                 7   20  11  3  5   4   16  5                                                  2   1   1   1  1   1   32  1                                                  1   1   2   2  1   24  1   27                                                 12  12  13  13 29  12  13  14                                                 14  31  29  28 28  30  10  10                                                 10  11  10  12 10  21  9   9                                                  30  31  11  23 14  19  18  19                                                 19  21  18  18 19  22  23  20                                                 22  21  20  22 22  20  16  26                                                 26  16  15  32 15  27  15  18                                                 17  17  25  17 17  24  25  16                                                 2   3   1   3  3   3   3   2                                                  3   2   3   4  4   3   3   3                                                  3   3   4   3  3   1   1   1                                                  1   2   1   1  1   1   1   1                                                  2   2   2   9  2   2   2   2                                                  2   6   6   6  6   6   9   6                                                  6   6   6   8  8   8   7   8                                                  7   7   7   7  5   5   4   4                                                  4   4   4   4  4   4   5   4                                                  6   5   5   5  5   5   5                                              valtbl[ ] = {                                                                         0   -1  1   -1 1   -2  0   2                                                  -3  3   -1  1  2   4   -4  -1                                                 1   -1  1   -1 1   -2  -6  -2                                                 5   2   -5  -3 -1  6   0   3                                                  1   1   1   -2 -7  2   -9  10                                                 -5  2   5   -1 -1  3   1   -10                                                -8  -11 7   8  9   -1  4   -1                                                 -13 4   -12 2  -1  1   -3  -1                                                 -2  1   1   4  -2  7   -1  -4                                                 6   17  -15 -14                                                                              11  12  -1  13                                                 14  15  -4  -6 -16 -1  -18 1                                                  -1  2   -2  -1 1   1   2   -2                                                 -1  1   -1  -1 1   -1  1   2                                                  3   -2  -2  -2 -3  2   2   3                                                  1   -1  2   1  2   2   -1  1                                                  -1  1   2   1  -2  -2  -1  2                                                  -1  -2  -1  1  2   -2  1   -1                                                 1   -2  2   1  1   -1  -2  -2                                                 2   1   1   -2 -1  1   -1  2                                                  -10 -4  -22 -6 -7  -9  -8  11                                                 -10 12  6   -8 -9  10  -3  9                                                  8   7   -7  5  -5  21  20  19                                                 -21 10  16  -17                                                                              -19 -20 18  22                                                 8   7   -7  - 2                                                                              -8  -9  -11 -12                                                9   6   5   4  3   -4  -3  -2                                                 -3  -5  2   3  2   -2  -5  -3                                                 5   4   3   -4 7   -7  9   8                                                  6   5   -5  3  -3  -4  6   -6                                                 -6  5   -6  4  3   -3  -5                                             }                                                                             __________________________________________________________________________

The next step in decoding is to convert the run/val pairs into an (8×8)block of DST coefficients, as follows: Define the scan path through an(8×8) matrix by the following numbers:

    ______________________________________                                        0       1      4       9    17    18   37    38                               2       3      8       10   19    25   39    45                               5       7      11      14   24    26   44    46                               6       12     13      15   27    32   47    53                               16      20     23      28   31    33   52    54                               21      22     29      30   34    35   55    60                               36      40     43      48   51    56   59    61                               41      42     49      50   57    58   62    63                               ______________________________________                                    

where the scan path is found by traversing these numbers in increasingorder. The (8×8) block of DST coefficients coeff[8][8] is created by thefollowing procedure:

    ______________________________________                                        for (i=0; i<8; i++)                                                             for (j=0; j<8; j++)                                                             coeff[i] [j] = 0;                                                         start at position "-1' on the scan path (one step "before"                    0)  for (each run/val pair)                                                     step forward by 'run' positions on the scan path                              deposit 'val' at the new position                                           }                                                                             ______________________________________                                    

The next step is to dequantize the block of coefficients. This is doneby applying quantization matrix number quant, as follows:

    ______________________________________                                        for (i=0; i<8; i++)                                                            for (j=0; j<8; j++)                                                            coeff[i] [j] = coeff[i] [j] << qmatrix[quant] [i] [j];                      ______________________________________                                    

The next step is to undo "DC prediction," which is used to furthercompress the DC coefficient coeff[0][0] in still blocks. If the blockbeing decoded is a still block (either because this is a still image, orbecause this block is part of the still strip in a relative image), DCprediction is undone by applying the following equations:

    coeff[0][0]+=prevDC

    prevDC=coeff[0][0]

The value of prevDC is initialized to 8*128 at the start of each imageplane.

The next step is to transform the (8×8) coefficient array into thespatial domain. This is done by applying an (8×1) DST to each of the 8rows and 8 columns of coeff[][]. The (8×1) DST can be described asfollows:

    ______________________________________                                        slant8x1(s,d,fwd)                                                                          // s = src array, d = dst array,                                 int s[ ],d[ ],fwd;                                                                         // fwd = 1 for forward xform, 0 for                                           // inverse                                                       int      r1,r2,r3,r4,r5,r6,r7,r8;                                             int      t,t1,*p;                                                             if       (fwd)                                                                {                                                                                      p = s;                                                                        r1 = *p++;                                                                    r2 = *p++;                                                                    r3 = *p++;                                                                    r4 = *p++;                                                                    r5 = *p++;                                                                    r6 = *p++;                                                                    r7 = *p++;                                                                    r8 = *p++;                                                                    SlantPart1;                                                                   SlantPart2;                                                                   SlantPart3;                                                                   SlantPart4;                                                                   p = d;                                                                        *p++ = r1;                                                                    *p++ = r4;                                                                    *p++ = r8;                                                                    *p++ = r5;                                                                    *p++ = r2;                                                                    *p++ = r6;                                                                    *p++ = r3;                                                                    *p++ = r7;                                                           }                                                                             else                                                                          {                                                                                      p = s;                                                                        r1 = *p++;                                                                    r4 =  *p++;                                                                   r8 = *p++;                                                                    r5 = *p++;                                                                    r2 = *p++;                                                                    r6 = *p++;                                                                    r3 = *p++;                                                                    r7 = *p++;                                                                    SlantPart4;                                                                   SlantPart3;                                                                   SlantPart2;                                                                   SlantPart1;                                                                   p = d;                                                                        *p++ = r1;                                                                    *p++ = r2;                                                                    *p++ = r3;                                                                    *p++ = r4;                                                                    *p++ = r5;                                                                    *p++ = r6;                                                                    *p++ = r7;                                                                    *p++ = r8;                                                           }                                                                             }                                                                             ______________________________________                                    

where butterfly(x,y) is the following operation:

    ______________________________________                                                   butterfly(x,y):                                                                  t = x+y;                                                                      y = x-y;                                                                      x = t;                                                          ______________________________________                                    

and SlantPart1, SlantPart2, SlantPart3, SlantPart4 are four macrosdefined as follows:

    ______________________________________                                        #define SlantPart1                                                              bfly(r1,r4);                                                                  bfly(r2,r3);                                                                  bfly(r5,r8);                                                                  bfly(r6,r7);                                                                #define SlantPart2                                                              bfly(r1,r2);                                                                  reflect(r4,r3);                                                               bfly(r5,r6);                                                                  reflect(r8,r7);                                                             #define SlantPart3                                                              bfly(r1,r5);                                                                  bfly(r2,r6);                                                                  bfly(r7,r3);                                                                  bfly(r4,r8);                                                                #define SlantPart4                                                              t = r5 - (r5>>3) + (r4>>1);                                                   r5 = r4 - (r4>>3) - (r5>>1);                                                  r4 = t;                                                                     #define reflect(s1,s2)                                                          t = s1 + (s1>>2) + (s2>>1);                                                   s2 = -s2 - (s2>>2) + (s1>>1);                                                 s1 = t;                                                                     ______________________________________                                    

The (8×1) DSTs are preferably performed in the following order: rowsfirst, then columns. (Doing columns followed by rows gives slightlydifferent, incorrect results.) After doing the (8×1) DSTs, all 64 valuesin the resulting (8×8) array are preferably right-shifted by 3 bits, andthen clamped to the range (-128, 127), if a delta block, or to the range(0, 255), if a still block.

If the block being decoded is a still block, no more processing isrequired. The DST calculation produces the block of reconstructed pixelsto be written to the image.

If the block being decoded is a relative block, the block ofreconstructed pixels is calculated as:

    ______________________________________                                        for (i=0; i<8; i++)                                                             for (j=0; j<8; j++)                                                            image[i] [j] = clamp0.sub.-- 255(prev[i] [j] + array[i] [j]);              ______________________________________                                    

where array[][] is the result of the DST calculation, prev[][] is the(8×8) block of pixels from the previous image, and clamp0₋₋ 255() is afunction that clamps a value to the range (0,255). The previous block isthe one in the same spatial location as the block in the current image,but offset by the motion vector for that block, which is eitherdetermined from the MotionVector array (if processing the Y plane) or is0 (if processing the U or V plane, or if FLAGS₋₋ MV==0).

During decoding the loop filter may need to be selectively applied. Ifthe FLAGS₋₋ FILTER flag is set, and if a block is not a still block, andif the quantization value for a block satisfies

    quant<=FilterThresh

and if the block is not empty (i.e., does not consist of only EOB), thenthe loop filter is applied to prev[] before adding the array[][] deltas.The preferred loop filter is a filter with kernel as follows: ##STR1##where the pixel marked x is replaced by:

    x=(a+b+c+d)>>2

where a,b,c,d are the four pixels in the corners of the (3×3) block. Onthe edges of an (8×8) block, a one-dimensional (1 0 1 ) kernel ispreferably used. The corner pixels of the block are preferably notfiltered.

Intra/Inter Decision Rules

A certain class of motion compensated video compression systems encodecertain blocks in motion compensated difference images as "intra" blocksand others as "inter" blocks. The decision to encode a block as an intraor inter block is based on a decision rule which is referred to as the"intra/inter decision rule". This section describes a preferred methodfor generating an intra/inter decision rule for conferencing system 100.The intra/inter decision rule generated by this method is (1)computationally simple, (2) encoded implicitly (requiring no bits fordifferentiating intra vs. inter blocks, (3) adaptive to spatiotemporalimage content, and (4) statistically optimal in providing a means ofdifferentiation between motion compensation artifacts and scenefeatures.

The conventional objective of encoding some blocks as intra in motioncompensated difference frames is to reduce the number of bits requiredto encode those blocks that have low spatial variation but high temporalvariation. The objective of encoding some blocks as intra in differenceframes is to reduce the effects of high frequency motion compensationartifacts (sometimes referred to as "mosquitoes" in the literature)without having to use (computationally expensive) loop filtering. Anarea in a motion compensated difference frame that exhibits mosquitoeswhen encoded as a quantized difference will instead appear blurred ifencoded as a quantized intra.

The preferred technique for generating an intra/inter decision rule fora given motion compensated video compression system works as follows:

Given:

1. A transform

2. A set of N quantizers for Inter blocks (Q1, Q2, . . . , QN)

3. A set of M quantizers for Intra blocks (K1, K2, . . . , KN)

4. A set of "training data" that is representative of the application inhand.

Let SAD(i,j) denote the "Sum of absolute differences" for block (i,j) ina motion compensated difference image.

Step 1:

For each Quantizer Qi, perform the following operation:

a. Compress the training data, using Qi as the quantizer for all theblocks in the all the motion compensated difference images.

b. By a visual observation of the (compressed and decompressed) trainingimage sequences, collect all blocks that contain perceptible mosquitoes.

c. From the set of blocks collected in (b), find the block with thelowest SAD. Denote the SAD of the block with the lowest SAD as LSADi(corresponding to quantizer Qi).

d. From the set of blocks collected in (b), select a subset of n blockswith the lowest SADs in the set.

e. For each block in the subset collected in (d), determine the numberof bits required to encode the block. Let B be the average number ofbits required to encode a block in the subset. For each intra quantizerKj, determine the average number of bits BKj required to encode a blockin the subset as an intra (using quantizer Kj). From the set {BK1, BK2,. . . , BKM}, find j such that B-BKj is minimized. Kj is the intraquantizer assigned to Qi.

Step 2:

From Step 1, for each Qi, there is a corresponding LSADi which is thelowest SAD value for which there are perceptible motion compensationartifacts and an intra quantizer Kj. The intra/inter decision rule isdefined as follows:

For each block (p,q) in a motion compensated difference frame, given aquantizer Qi (as determined by an external quantizer selection process)the block is encoded as intra if and only if SAD(p,q)>LSADi. Intraquantizer Kj is used to encode the block.

A major advantage of the intra/inter decision rules generated by thistechnique is that the intra/inter decision is implicit in the method andis known to both the encoder and decoder. Therefore, it does not need tobe explicitly transmitted and thus requires no bits.

Post Reconstruction Loop Filtering

This section describes a preferred method of "loop filtering" forconferencing system 100 for the reduction of high frequency artifactsassociated with motion compensated video compression for the presentinvention. A traditional loop filtering operation operates on thepreviously decoded (reference) image. Certain blocks of the previouslydecoded image are low-pass filtered prior to motion compensation. Thisreduces the high frequency content in the reference block and, as aresult, the high frequency content in the final output.

In the preferred method of loop filtering, a low-pass filter is appliedto certain blocks after the motion compensation and addition operationto generate a filtered reconstructed image. This approach to loopfiltering has two major advantages:

1. It is easier to implement, since the motion estimation anddifferencing operations may be merged into one operation.

2. It has a greater low-pass filtering effect on the reconstructed imagesince the final image is filtered instead of the reference image only.

Adaptive Loop Filter Switching Criteria

This section describes a preferred method for generating a criterion forthe switching ("on" or "off") of a loop filter in conferencing system100. The loop filter switching criterion generated by this method isbetter adapted to the spatiotemporal image content and provides adifferentiation between motion compensation artifacts and scenefeatures. A traditional loop filtering operation operates on thepreviously decoded (reference) image. Certain macroblocks (typically16×16 areas) of the previously decoded image are low-pass filtered priorto motion compensation. This reduces the high frequency content in thereference macroblock and, as a result, the high frequency content in thefinal output.

The objective of loop filtering is to reduce high frequency artifactsassociated with residual quantization noise in motion compensateddifference images. Ideally, only those macroblocks should be filteredthat exhibit such motion compensation artifacts. A criterion fordeciding whether or not a given macroblock should be loop filtered ornot is referred to as the "loop filter switching criterion."

A conventional loop filter switching criterion is to apply a loop filterif the macroblock has a non-zero motion vector and not to apply it ifthe motion vector for the given macroblock is the zero vector. A majordrawback of this criterion is that it filters macroblocks that havenon-zero motion but no motion compensation artifacts.

The preferred method for generating a loop filter switching criterionworks as follows:

Given:

1. A transform

2. A set of N Quantizer (Q1, Q2, . . . , QN )

3. A set of representative "training data" for the application at hand.

Let SAD(i,j) denote the "Sum of absolute differences" for Macroblock(i,j) in a motion compensated difference image.

Step 1:

For each Quantizer Qi, perform the following operation:

a. Compress the training data, using Qi as the quantizer for all themacroblocks in the all the motion compensated difference images.

b. By a visual observation of the (compressed and decompressed) trainingimage sequences, collect all macroblocks that contain perceptible highfrequency motion compensation artifacts (sometimes referred to as"mosquitoes" in the literature).

c. From the set of macroblocks collected in (b), find the macroblockwith the lowest SAD. Denote the SAD of the macroblock with the lowestSAD as LSADi (corresponding to quantizer Qi).

Step 2:

From Step 1, for each Qi, there is a corresponding LSADi which is thelowest SAD value for which there are perceptible motion compensationartifacts. The loop filter switching criterion is defined as follows:

For each Macroblock (p,q) in a motion compensated difference frame,given a quantizer Qi (as determined by an external quantizer selectionprocess) the loop filter is applied if only if SAD(p,q)>LSADi.

Design of Quantization Tables

This section describes a preferred method for designing quantizationtables to be used for quantization in conferencing system 100. Thispreferred method exploits the perceptual properties of the human visualsystem in a statistical sense to arrive at quantization tables thatminimize perceived quantization artifacts at a given effective bit rate.

In conventional video compression systems, the quantization process isspatially adaptive. Different regions in the image are quantized usingdifferent quantizers. In a transform-based video compression system thatuses linear quantization, the quantization operation may be completelyspecified by a table of numbers, each of which corresponds to the(linear) quantizer step size to be used to quantize a specific frequencyband in the transform domain.

The present invention relates to the design of the quantization tableQ[8][8] for conferencing system 100. The design process is as follows:

Given:

1. Transform-based conferencing system 100

2. A set of video sequences that are representative of the applicationat hand

3. A specification of target bitrate (or compression ratio) for theapplication.

Objective:

To design a set of N quantization tables Q1, Q2, . . . , QN such that:

a. QN/2 results in target bitrate for typical video sequences.

b. Q1, . . . , QN meet a specified dynamic range specification. For agiven video sequence, the bitrate generated using Q1 should be about Ktimes the bitrate generated by QN. Here K is the dynamic rangespecification and is usually dependant on the variability of theallocated channel bandwidth of the channel over which the compressedvideo bitstream is being transmitted.

c. Q1, . . . , QN minimize the perceived artifacts in the processed(compressed and decompressed) video sequence at their point of operation(in terms of bit rate).

Procedure:

Step 1. Design of Q1

Q1 is the weakest quantizer table and is designed so as to generate noperceptible artifacts at the expense of a bitrate that is potentiallymuch higher than Target Bitrate. Q1 is designed as follows:

    Set Q[i][j]=1 for all i,j (all frequency bands)

Starting from the lowest frequency band to the highest frequency band,

For each band (i,j),

a. Increment Q[i][j]

b. Use Q[8][8] as the quantizer in the given video compression system

c. If there are any perceivable artifacts in the processed videosequence,

i. Decrement Q[i][j]

ii. Goto the next band Else goto (a)

The above process generates a quantizer table (Q1) that is at theperceptual threshold, referred to as the perceptual threshold quantizer(PTQ).

Step 2. Design of Q2, Q3, . . . , QN/2

Let B1 be the bitrate generated using quantizer Q1 with a typical videosequence. Let BT be the target bitrate. The objective now is to designQ2, Q3, . . . QN/2 such that QN/2 generates target bitrate (BT) fortypical sequences and Q2, Q3, . . . , QN/2-1 generate monotonicallydecreasing intermediate bitrates between B1 and BT. From the perspectiveof a bitrate controller, it is desirable to have a linear decrease inbitrate with quantizer table index. Tables Q2, Q3, . . . , QN/2 aredesigned with this requirement in mind. The following is the designprocedure for tables Q2,Q3, . . . , QN/2:

Let dB=(B1-BT)/(N/2).

Set Q2=Q1

For each quantizer Qk, k=2 to N/2

Starting from the highest frequency band to the lowest frequency band,For each band (i,j)

a. Set Qk=Qk-1

b. Increment all Qk[i][j] with the same horizontal or vertical frequency

c. Use Qk[8][8] as the quantizer in the given video compression system

d. If the bitrate is reduced by dB,

i. Save the state of Qk[8][8]

ii. Goto the next band at 1 Else goto 2.

Amongst the quantizer states saved in (d)(i), select that quantizer thathas the least perceptible artifacts for typical video. This is thechoice for Qk.

Step 3. Design of QN/2+1, . . . , QN.

From the perspective of a bitrate controller, it is desirable to have aprogressively increasing decrease in bitrate with quantizer table indexfrom table N/2+1 to table N. The design of tables QN/2+1, . . . , QN isthe same as the design for tables 2, . . . , N/2 except that for eachnew table, dQ increases instead of remaining constant. The magnitudes ofthe dQs for quantizers QN/2+1, . . . , QN depend on the desired dynamicrange in bitrate and the manner of decrease in bitrate with quantizertable index. For example, if the desired dynamic range is BT to BT/4from QN/2 to QN and the decrease in bitrate is logarithmic then

    ______________________________________                                        dQ(N/2+1) = dQ(N/2)                                                           for i=(N/2+2) to (N/2)                                                          dQi = kdQi-1                                                                dQ(N/2+1) + dQ(N/2+2) + . . . + dQN = BT - BT/4                               dQ(N/2) (1 + k + k*k + k*k*k + . . .) = 3BT/4                                 (1 + k + k*k + k*k*k + . . .) = 3BT/4 / (dQN/2)                               (1+2+3+4+. . . +(N/2-1)) logk = log (3BT/4 / dQN/2)                           logk = log (3BT/4 / dQN/2) / N/4                                              k = (3BT/4 / dQN/2) to the power 4/N                                          ______________________________________                                    

Adaptive Transform Coefficient Scanning

This section describes a preferred method of transform coefficientscanning in conferencing system 100, a transform-based image and videocompression system, that exploits the properties of the transform andthe associated quantization technique to generate coefficient scanorders that generate the lowest bitrates. The image (for imagecompression) or motion compensated difference (for motion compensatedvideo compression) is transformed. The transformed coefficients arequantized. The transformed quantized coefficients are scanned in acertain order from a two dimensional array to a one dimensional array.This one dimensional array is re-represented by a run-length--value (RV)representation. This representation is then entropy coded and the resulttransmitted or stored to be decoded.

The preferred method applies to the "scan" part of the processing wherethe quantized transformed coefficients are scanned from a twodimensional array to a one dimensional array. The purpose of thisscanning is to facilitate efficient representation by a RVrepresentation. The same scan-order is applied to every block in therepresentation.

The preferred method of scanning involves the following operations:

Given:

1. A transform.

2. A set of N quantizers (typically quantization matrices) denoted byQ1, Q2, . . . , QN.

3. Representative "training" data for the target application.

Step 1.

For each quantizer Qi, generate quantized transformed blocks for all ofthe training data.

Step 2.

Compute the average amplitude for each of the transform coefficientsfrom the quantized transformed blocks for all the training data.

Step 3.

Sort the average amplitudes computed in Step 2.

Step 4.

For quantizer Qi, the scan order Si is generated by the locations of the(amplitude sorted) coefficients from Step 3. The largest coefficient isthe first in the scan order and the smallest is the last.

Using this preferred method, a scan order Si is generated for eachquantizer Qi. In the encode and decode process, for each block for whichQi is used as the quantizer, Si is used as the scan order.

The advantage of this invention over previous scanning techniques isthat due to the adaptive scan orders, the RV representations are moreefficient and for a given quantizer, fewer bits are required to encode agiven block than with conventional non-adaptive zigzag scanning.

Spatially Adaptive Quantization

This section describes a preferred method of spatially adaptivequantization for conferencing system 100. The preferred method providesa means of efficiently encoding motion compensated difference images. Aconventional non-adaptive quantization technique simply takes a givenquantizer for each frame and applies that quantizer uniformly to everymacroblock (16×16 area) in the image. An adaptive quantization techniqueapplies different quantizers to different macroblocks in a given frame.Information about which quantizer has been applied to which block isalso encoded and transmitted.

The preferred method of spatially adaptive quantization is based on the"sum of absolute difference" (SAD) that has already been computed foreach macroblock by the motion estimation subroutine. The preferredquantizer selection method works as follows:

Step 1.

The mean SAD for the entire frame is computed. This denoted by MSAD.

Step 2.

For each macroblock, if the SAD of the macroblock is lower than themean, then it is assigned a finer quantizer than the mean quantizer(which is the global quantizer for this frame passed down by thebit-rate controller). Conversely, if the SAD in the macroblock is higherthan the mean, then it is assigned a coarser quantizer.

In a case where there are 16 quantizers, numbered 1 through 16 withhigher numbers denoting finer quantizers, let SAD(i,j) be the SADassociated with the current macroblock (i,j). Let MSAD be the mean SADin the frame. Let Q(i,j) denote the quantizer assigned to the currentmacroblock. Let QG denote the global quantizer for the frame. ThenQ(i,j) is assigned as:

    Q(i,j)=QG+8*log2 ((SAD(i,j)+2MSAD)/(2SAD(i,j)+MSAD))

Q(i,j) is saturated to the range (1,16) after performing the aboveoperation.

There are 2 major advantages of the preferred spatially adaptivequantization technique over conventional techniques:

1. The spatial adaptation is based on values that have already beencomputed in the motion estimation routine. Therefore the spatialadaptation process is computationally simple.

2. The spatial adaptation process generates an optimal quality imagegiven the bit-budget of the current frame by distributing bits todifferent macroblocks in proportion to the perceived effect ofquantization on that macroblock.

Fast Statistical Decode

Host processor 202 preferably performs fast statistical decoding. Faststatistical decoding on host processor 202 allows time efficientdecoding of statistically coded data (e.g., Huffman decoding). Moreover,since statistical Huffman coding uses code words that are not fixed(bit) length, the decoding of such codewords is generally accomplishedone bit at a time. The preferred method is as follows:

1. Get next input bit and juxtapose with bits already in potentialcodeword (initially none).

2. If potential codeword is a complete codeword, then emit "symbol",eliminate bits in potential codeword, and go to (1). Otherwise, ifpotential codeword is not a complete codeword, then go to (1).

The preferred method of the present invention provides decoding of one"symbol" in one operation, as follows:

a. Get next (fixed number) several input bits.

b. Use the input bits to select a symbol and emit symbol.

c. Go to (a).

The statistical code used is designed to be "instantaneous," which meansthat no codeword "A" is a "prefix" of any codewords "B". This allows alookup table to be constructed which may be indexed by a potentialcodeword, unambiguously yielding a symbol corresponding to the codeword.The potential codeword is guaranteed to contain a complete codewordsince it starts with a codeword, and it is as long as the longestcodeword.

Contrast, Brightness, and Saturation Controls

This section describes a preferred integer implementation of contrast,brightness, and saturation controls for the present invention foradjusting and for application of the controls to realtime video. Theimplementation has two pans. The first is a method of generatingtranslation tables to implement adjustable brightness, contrast, andsaturation controls. The second is a method of using the tables tochange the appearance of video being displayed.

The generation of the tables uses integer operations in the generationof tables that express floating point relations. Prior to application ofany controls, the video data consists of a description of the Y, V, andU components at 8 bits per value. The problem is to provide atranslation from the decoded Y values to Y values that reflect thecurrent setting of the brightness and contrast controls, and further toprovide a translation from the decoded U and V values to U and V valuesthat reflect the current setting of the saturation control.

The method begins with an identity translation table (f(x)=x). Ascontrols are changed, the identity translation becomes perturbedcumulatively. In the case of brightness, control changes are indicatedby a signed biased value providing both direction and magnitude of thedesired change. The current translation table are changed into f(x)=x-k,for x>=k, and f(x)=0 for 0<=x<k (decrease) or f(x)=x+k, for x<=255-k,and f(x)=255 for 255>=x>255-k (increase).

In the case of contrast, control changes are indicated by a scaledfractional value. The value indicated "n" represents "(n+1)/SCALE"change: a "change" of (SCALE-1) yields no change, a change of (SCALE)yields a change by 1/SCALE in each of the translation table values. Thedefinition of contrast as y'=(n*(y-128))+128 (for 8 bit values) is thenprovided by subtracting 128 from the translation table value,multiplying by SCALE, multiplying by the indicate control change value,and then dividing by SCALE twice to remove the scale multiple implied inthe representation of the control change value, and the multiplyexplicitly performed here. 128 is then added to the modified translationtable value and the result is clamped to the range of 0 to 255inclusive.

This method avoids the use of floating point arithmetic in thecomputation of the proper translation table values. In the definitionoffered of "contrast" the value "n" is a floating point number.Saturation is simply contrast as applied to the chrominance data, and ishandled in the same way as the contrast control, but with a differentcopy of the translation table.

The translation tables are made available to the host processor in thesame locale as the data that they are used to translate: aftergeneration of the modified translation tables, the tables are appendedto the data area for the luminance and chrominance, at known fixedoffsets from the start of same data areas (on a per instance basis, eachvideo window has its own copy of this data. ) This allows the hostprocessor to access the translation tables with a 1 processor clockpenalty in address generation (for an Intel® 486 microprocessor; thereis no penalty on an Intel® Pentium® processor), and with a high degreeof locality of reference, and no pointer register reloads (due to thefixed offset.)

The translation of the decoded Y, V, and U values is performed byreading and translating eight values and then writing the eighttranslated values as two 32-bit values to the destination. This isimportant to Intel® architecture microprocessors, and in particular isimportant to the Intel® 486 processor, which usually runs with a writesaturated bus.

For the method of performing the translation, the BX register is assumedto contain zeroes in the high order 8(24) bits. The low order 8 bits areloaded with the value to translate, and the value is used as the baseregister with an index register (set to the offset of the translationtable+base of data buffer) in an indirect load to accomplish thetranslation. The destination of the load is changed as the operation isrepeated over multiple values, until register storage is exhausted, atwhich point the translated values are written out and the cycle repeats.The process here described executes at a sustained three or four clocksper value translated.

Audio Protocol

Referring now to FIG. 26, there is shown a representation of acompressed audio packet for conferencing system 100, according to apreferred embodiment of the present invention. Source audio is audiothat is captured (and optionally monitored) at the local system and sentto the comm subsystem for transmission. Sink audio is audio that isreceived from the comm subsystem for playback on the local system. Audiois preferably handled on audio/comm board 206 and not on host processor202. The compressed audio packet of FIG. 26 is that which is actuallysent/received from the communications subsystem and not necessarily thatmanipulated by an application on the host processor. The audio packetfields are defined as follows:

    ______________________________________                                        Timestamp                                                                             Value used to synchronize audio and video frames at                           the receive endpoint. The audio stream preferably                             generates timestamps as a master clock that are                               copied to the captured video frames before                                    transmission.                                                         Reserved                                                                              Reserved field.                                                       Mute    Bit indicates whether or not the audio stream is                              muted or not. The audio is muted when the bit is set.                         When the Mute bit is set, no audio data is sent.                      Data    Compressed audio data.                                                ______________________________________                                    

The length of the audio data is not explicitly specified in the packetheader. A receiving endpoint's comm subsystem reassembles an audiopacket and therefore implicitly knows the length and can report it toits application. The length of an audio packet is a run-time parameterand depends on the compression method and the amount of latency desiredin the system. The preferred audio compression/decompression methodimplementation has 100 msecond latency, which translates to 200 bytes ofcompressed audio data per packet.

Compressed Audio Bitstream

The preferred audio stream for conferencing system 100 is a modificationof the European Groupe Speciale Mobile (GSM). GSM was developed in thecontext of the standardization of the European digital mobile radio. Itresulted from the combination of the Regular-PulseExcitation/Linear-Predictive-Coding codec developed by Philips (Germany)with the Multi-Pulse-Excitation/Linear-Predictive-Coding codec devisedby IBM (France). For further information, see the ETSI-GSM TechnicalSpecification, GSM 06.10, version 3.2.0, UDC 621.396.21, published bythe European Telecommunication Standards Institute in Valbonne Cedex,France.

The data rate of the standard GSM codec is 13.0 kbits/sec. The preferredGSM implementation for conferencing system 100 has a bit rate of 16kbits/sec. The mean opinion score (MOS) quality rating of the preferredGSM implementation is 3.54. It is not prone to rapid quality degradationin the presence of noise. The relative complexity is about 2 MOPSs/s.Due to implementation processing considerations, the standard GSMimplementation is adjusted to yield the preferred GSM implementation. Inaddition, headers are added to provide extra control information, suchas frame counting and muting.

In order to save processing, the 260-bit audio frame is not packed. Thisresults in a 320-bit flames. These flames occur every 20 mseconds. Thisincreases the bit rate from 13 kbits/sec to 16 kbits/sec. Thecomposition of the preferred audio frame is as follows:

    ______________________________________                                        typedef struct {                                                                        unsigned int lar1:                                                                         6;     /* stp parameters */                                      unsigned int lar2:                                                                         6;                                                               unsigned int lar3:                                                                         5;                                                               unsigned int lar4:                                                                         5;                                                               unsigned int lar5:                                                                         4;                                                               unsigned int lar6:                                                                         4;                                                               unsigned int lar7:                                                                         3;                                                               unsigned int lar8:                                                                         3;     }  STP;                                         typedef struct {                                                                        unsigned int lag                                                                           7;                                                               unsigned int gain                                                                          2;     /* ltp parameters */                                      unsigned int grid                                                                          2;     /* rpe parameters */                                      unsigned int xmax                                                                          6;                                                               unsigned int x0                                                                            3;     /* pulse amplitude */                                     unsigned int x1                                                                            3;                                                               unsigned int x2                                                                            3;                                                               unsigned int x3                                                                            3;                                                               unsigned int x4                                                                            3;                                                               unsigned int x5                                                                            3;                                                               unsigned int x6                                                                            3;                                                               unsigned int x7                                                                            3;                                                               unsigned int x8                                                                            3;                                                               unsigned int x9                                                                            3;                                                               unsigned int x10                                                                           3;                                                               unsigned int x11                                                                           3;                                                               unsigned int x12                                                                           3;     }  LTP.sub.-- RPE                               typedef struct {                                                                        STP       frame;                                                              LTP.sub.-- RPE                                                                          subframe(4);                                                                             }  GSMBITS;                                    ______________________________________                                    

The result of not packing these structs on a Texas Instruments® C31 DSP,a 32-bit processor, is a 320-bit frame. At a frame rate of 50frames/sec, the data rate is 16.0 kbits/sec.

A header has also been added to groups of frames. The length of theheader is one 32-bit word. The MSB is a mute flag (1=mute). Theremaining bits represent a timestamp. This time stamp is not actuallytime, but is preferably a frame counter. The initial value of it isarbitrary. It is therefore a relative number representing the progressof audio frames and useable for synchronization.

Data Protocol Data packets are inside TII packets. The data conferencingapplication will have its own protocol inside the TII protocol stack.

Communication-Level Protocols

The application-level audio, video, and data packets described in theprevious section are sent to the comm subsystem for transmission to theremote site. The comm subsystem applies its own data structure to theapplication-level packets, which the comm subsystem treats as genericdata, and defines a protocol for transport. In a preferred embodiment ofthe present invention, the basic transport is unreliable. That is, atthe basic level, there is no guarantee that application data will reachthe destination site and, even if it does, there is no guarantee as tothe correctness of the data delivered. Some applications will use theunreliable communication services, such as audio and video. Forapplications requiring guaranteed delivery of data, reliability is builton the basic unreliable service. Application data is an example of adata type requiring reliable transport; control information between peerprocesses is another.

Reliable Transport Comm Protocols

Referring now to FIG. 27, there is shown a representation of thereliable transport comm packet structure, according to a preferredembodiment of the present invention. For reliable transport,conferencing system 100 preferably uses a protocol akin to LAPB. Sincetransport is preferably on ISDN B-channels, which are assumed to havealready been set up, there is no need to include those portions of LAPBthat deal with circuit establishment and teardown (e.g. SABM, FRMR, UA,and DISC). Therefore, the preferred reliable transport comm protocol isvoid of those portions. The fields of the preferred reliable transportcomm packet are defined as follows:

    ______________________________________                                        Control                                                                              Defines the type of packet and relays acknowledgment                          information. The types of packets are: Information (I),                       Receiver Ready (RR), Receiver Not Ready (RNR),                                and Reject (REJ).                                                      Length Length of the client data portion of the packet, in                           bytes.                                                                 CRC    Cyclic redundancy check code.                                          Data   Client data of length specified by the Length field.                   ______________________________________                                    

For an Information (I) packet, the format of the control field is asfollows:

    ______________________________________                                        (Bit)       0        1-3      4      5-7                                      (Field)     0        NS       P      NR                                       ______________________________________                                    

The NS bit field is used to refer to a send sequence number. NS isinterpreted as specifying to the receiving site the next packet to besent. The NR bit field is used to refer to a receive sequence number. Itis used to acknowledge to a sender that the receiver has received packetNR-1 and is expecting packet NR. The P bit field is the LAPB poll bitand is are not used in the preferred embodiment. All sequence numbersare modulo-8 meaning that at most 7 packets can be outstanding. It isthe responsibility of the transmitting sites to assure that they do nothave more than 7 packets outstanding. An Information packet is used tosend client data. The receive acknowledgment can be piggybacked on inthe NR bit field.

The Receiver Ready (RR), Receiver Not Ready (RNR), and Reject (REJ)packets are supervisory packets that are used for acknowledgment,retransmission, and flow control. They are not used to carry clientdata.

For a Receiver Ready (RR) packet, the format of the control field is asfollows:

    ______________________________________                                        (Bit)     0      1       2     3    4     5-7                                 (Field)   1      0       0     0    PF    NR                                  ______________________________________                                    

The PF bit field is the LAPB poll/final bit and is not used in thepreferred embodiment. The RR packet is used in two cases. The first caseis to acknowledge packet receipt when there are no packets pendingtransmission on which to piggyback the acknowledgment. The second caseis when the link is idle. In this case, an RR packet is sentperiodically to assure the remote site that the local site is stillalive and doing well.

For a Receiver Not Ready (RNR) packet, the format of the control fieldis as follows:

    ______________________________________                                        (Bit)     0      1       2     3    4     5-7                                 (Field)   1      0       1     0    PF    NR                                  ______________________________________                                    

The RNR packet is sent by a receiver to indicate to the remote site thatthe remote site should stop sending packets. Some condition hasoccurred, such as insufficient receive buffers, rendering the remotesite unable to accept any further packets. The RNR packet is intended tobe used for temporary flow control. When the remote site is able toaccept more packets it issues an RR frame.

For a Reject (REJ) packet, the format of the control field is asfollows:

    ______________________________________                                        (Bit)     0      1       2     3    4     5-7                                 (Field)   1      0       0     1    PF    NR                                  ______________________________________                                    

The REJ packet is sent as a form of negative acknowledgment. Thereceiver of an REJ packet interprets the NR bit field as a request toretransmit all packets from NR to the most currently sent, inclusive.

Unreliable Transport Comm Protocols

At the lowest layer of conferencing system 100, an unreliable protocolis preferably used to transport data on the ISDN B-channels. For thoseapplications requiring reliability, the reliable protocol discussed inthe previous section is added on top of the unreliable protocoldiscussed in this section. The unreliable protocol sits atop of HDLCframing which the unreliable protocol uses for actual node-to-nodetransport of packets. Even though HDLC framing is used, a data linkprotocol is not implemented. In particular, there is no guarantee thatdata packets will be delivered or that they will be uncorrupted at thereceive node of a link. The CRC validation of the HDLC is used to detectcorrupted data.

The unreliable protocol provides for logical channels and virtualizationof the two Basic Rate ISDN B-channels. Logical channels are local siteentities that are defined between the DLM and TII is layer and theclient (i.e., application program) using them. The logical channelsprovide the primary mechanism clients use to send multiple data types(e.g., audio, video, data). The layer services multiplex these datatypes together for transmission to the remote sites.

In a preferred embodiment, logical channel zero is used as a controlchannel. Site peers (i.e., two conferencing systems in a conferencingsession) use this control channel to exchange information on their useof other logical channels. Logical channels are half-duplex. Therefore,two channels are necessary to send and receive data. A priorityattribute is associated with a logical channel (and therefore with adata type). The unreliable protocol asserts that higher priority datawill always be sent ahead of lower priority data when both are pending.Priorities are assigned by an API call to the TII services. Audio hasthe highest priority, then data, and last video.

Although the ISDN Basic Rate Interface (BRI) defines two physical 64kbit/second B-channels for data, the services at both DLM and TIIvirtualize the separate B-channels as a single 128 kbit/second channel.Client data types, defined by their logical channels, are multiplexedinto a single virtual stream on this channel. In a preferred embodiment,this inverse multiplexing is accomplished by breaking all packets intoan even number of fragments and alternating transmission on the twophysical B-channel connections. Initially, after channel establishment,the first fragment is sent on the B1-channel, the second on theB2-channel, etc. At the receiving site, fragments are collected forreassembly of the packet.

Referring now to FIG. 28, there is shown a representation of theunreliable transport comm packet structure, according to a preferredembodiment of the present invention. The fields of the preferredunreliable transport comm packet are defined as follows:

    ______________________________________                                        Flag   Standard HDLC Flag field.                                              DestID The receiving site's logical channel identifier. The                          transmitting site peer acquires this ID by communi-                           cating to the remote site before exchanging data. This is                     done using a control logical channel (i.e., channel                           zero).                                                                 SrcID  The sending site's logical channel identifier. The type                       of data in the packet can be determined by knowing the                        logical channel ID-to-data type mapping. The current                          implementation uses the following mapping: The                                mapping is from DLM channels to TII channels, which                           occur at the TII level. At the time the TII channel is                        opened for a datatype, TII dynamically assigns unique                         DLM channels for different data types in ascending                            order starting from one (1).                                           PktNo  The packet sequence number. Distinguished from the                            FragNo field which counts the fragments within a                              packet. The PktNo field is used by the receiving site                         peer to implement a sliding window protocol. This                             allows packet buffering which is used to compensate                           for transmission delays.                                               SOP    If the SOP bit is set, then the current fragment is the                       start of a packet.                                                     EOP    If the EOP bit is set, then the current fragment is the                       end of a packet.                                                       Rsvd   Reserved field.                                                        FragNo The fragment sequence number. Distinguished from the                          PktNo field which counts the number of whole packets.                         The FragNo is used by the receiving site peer to                              reassemble fragments into packets. The SOP and EOP                            fields are used to locate the start and end of a whole                        packet, respectively.                                                  Data   The data field.                                                        CRC    Standard HDLC CRC field.                                               Flag   Standard HDLC Flag field.                                              ______________________________________                                    

Data Structures, Functions, and Messages

This section contains the data structures and definitions of thefunctions and messages for conferencing API 506, video API 508, audioAPI 512, and comm API 510.

Conferencing API Data Structures, Functions, and Messages

Conferencing API 506 utilizes the following data types:

    ______________________________________                                        LPHCALL       Pointer to a call handle.                                       LPAVCB        Pointer to an Audio Video Control                                             Block (AVCB).                                                   LPCCB         Pointer to a Configuration Control                                            Block (CCB).                                                    LPBITMAPINFO  Pointer to a Microsoft ® Windows                                          BITMAPINFO structure that defines a                                           DIB (Device-Independent Bitmap).                                LPHSTGRP      Pointer to the handle of a stream group.                        LPABBUSCARDINFO                                                                             Pointer to a ABBUSCARDINFO,                                                   which defines the personal card infor-                                        mation, from Address Book. Contains                                           business card information; format is                                          specified by the GUI.                                           ______________________________________                                    

Conferencing API 506 utilizes the following structures that are passedto conferencing API 506 in function calls (e.g., CF₋₋ Init, CF₋₋ CapMon)and then passed by conferencing API 506 to the audio/video managers:##SPC2##

ARegisterMonitor (asynchronous)

This function registers an audio stream monitor. The Audio Managermaintains a packet count on each open stream. This count represents arunning clock where the elapse time since the initiation of the audiostream is simply the packet count times the latency represented by eachpacket. Initiation here refers to the moment a local audio stream entersthe AST₋₋ CAPTURE state. Users of the audio subsystem gain access tothis clock source via an audio stream monitor. ##SPC3##

Data Exchange

All the data communication is done in "message passing" fashion. Thismeans that a send satisfies a receive on a specific channel, regardlessof the length of the sent data and the receive buffer length. If thelength of the sent message is greater than the length of the postedreceive buffer, the data is discarded. All these calls are"asynchronous", which means that the data in the send buffer is notchanged until a "data-sent" event has been sent to the application, andthe contents of receive buffer are not valid until a "received-complete"event has been detected for that channel.

    __________________________________________________________________________    SendData       Sends data to peer. If there are no receive buffers                           posted on the peer machine, the data will be lost.             TSTATUS SendData (HCHAN hChan, LPSTR Buffer, WORD Buflen, DWORD                  TransID)                                                                   hChan:         Handle to channel opened via OpenChannel.                      Buffer:        A pointer to the buffer to be sent.                            Buflen:        The length of the buffer in bytes.                             TransID:       This is a user defined transaction ID which will be                           passed to the local channel handler along with the                            status message to identify the transaction.                    Return values:                                                                SUCESSFUL      Data queued for transmission.                                  CHAN.sub.-- INVALID                                                                          Invalid channel handle.                                        CHAN.sub.-- TRANFULL                                                                         Channel transaction table full.                                Status Messages:                                                              CHAN.sub.-- DATA.sub.-- SENT                                                                 Tells the application that the data has been                                  extracted from the buffer and it is available for                             reuse.                                                         CHAN.sub.-- DATA.sub.-- LOST                                                                 This message will be delivered to the caller if                               the data could not be sent.                                    Peer Messages:                                                                CHAN.sub.-- DATA.sub.-- LOST                                                                 This message will be delivered to the peer if an                              adequate ReceiveData buffer is not posted.                     CHAN.sub.-- RECV.sub.-- COMPLETE                                                             Indicates that data was received.                              ReceiveData    Data is received through this mechanism. Nor-                                 mally this call is issued in order to post receive                            buffers to the system. When the system has                                    received data in the given buffers, the Channel                               Handler will receive a                                                        "CHAN.sub.-- RECV.sub.-- COMPLETE" notification.               TSTATUS ReceiveData (HCHAN hChan, LPSTR Buffer, WORD Buflen,                     DWORD TransID)                                                             hChan:         Handle to channel handle opened via                                           AcceptChannel.                                                 Buffer:        A pointer to the buffer to be filled in.                       Buflen:        The length of the buffer in bytes. Max. bytes to                              receive.                                                       TransID:       This is a user defined transaction ID which will be                           passed to the channel handler along with the status                           message to identify the transaction. This ID and                              the number of bytes actually received are returned                            as part of the CHAN.sub.-- RECV.sub.-- COMPLETE                               notification.                                                  Return values:                                                                SUCESSFUL      Receive buffer was posted.                                     CHAN.sub.-- INVALID                                                                          Invalid channel handle.                                        CHAN.sub.-- TRANFULL                                                                         Channel transaction table full.                                Status Messages:                                                              CHAN.sub.-- RECV.sub.-- COMPLETE                                                             Indicates that data was received.                              CHAN.sub.-- DATA.sub. -- LOST                                                                This message will be delivered if the buffer is                               inadequate for a data message received from the                               peer.                                                          Peer Messages:                                                                none                                                                          Communications Statistics                                                     GetTIIStats    Return statistics for the TII subsystem. See                                  TII.sub.-- STATS structure for details.                        TSTATUS FAR PASCAL.sub.-- export GetChanStats (IN BOOL bResetFlag OUT            LP.sub.-- TII.sub.-- STATS lpTiiStats)                                     bResetFlag:    Boolean Reset statistics if true.                              lpTiiStats:    Pointer to the TII.sub.-- STATS structure.                     Return values: none                                                           Status Messages:                                                                             none                                                           Peer Messages: none                                                           GetChanStats   Return statistics for the given Channel. See                                  CHAN.sub.-- STATS structure for details.                       TSTATUS FAR PASCAL.sub.-- export GetChanStats(IN HCHAN hChan, IN BOOL            bResetFlag, OUT LP.sub.-- CHAN.sub.-- STATS lpChanStats)                   hChan:         Channel handle                                                 bResetFlag:    Boolean reset statistics if true.                              lpChanStats:   Pointer to the CHAN.sub.-- STATS structure.                    Return values:                                                                CHAN.sub.-- INVALID                                                                          The channel handle was invalid.                                Status Messages:                                                                             none                                                           Peer Messages: none                                                           GetChanInfo    This function will return various statistical infor-                          mation about a channel (e.g., priority, reliability).          TSTATUS GetChanInfo (HCHAN hChan, LPCHAN.sub.-- INFO lpChanInfo)              hChan:         Handle to channel                                              lpChanInfo:    Pointer to channel info (to be returned by the                                call).                                                         Return values:                                                                CHAN.sub.-- INVALID                                                                          Invalid channel handle.                                        Status Messages:                                                                             none                                                           Peer Messages: none                                                           __________________________________________________________________________

Alternative Embodiments

In a preferred embodiment of conferencing system 100, video encoding isimplemented on video board 204 and video decoding is implemented on hostprocessor 202. In an alternative preferred embodiment of the presentinvention, video encoding and decoding are both implemented on videoboard 204. In another alternative preferred embodiment of the presentinvention, video encoding and decoding are bother implemented on thehost processor.

In a preferred embodiment of conferencing system 100, audio processingis implemented by audio task 538 on audio/comm board 206. In analternative preferred embodiment of the present invention, audioprocessing is implemented by Wave driver 524 on host processor 202.

In a preferred embodiment, conferencing systems 100 communicate over anISDN network. In alternative preferred embodiments of the presentinvention, alternative transport media may be used such as Switch 56, alocal area network (LAN), or a wide area network (WAN).

In a preferred embodiment, two conferencing systems 100 participate in aconferencing session. In alternative preferred embodiments of thepresent invention, two or more conferencing systems 100 may participatein a conferencing session.

In a preferred embodiment, the local sources of analog video and audiosignals are a camera and a microphone, respectively. In alternativepreferred embodiments of the present invention, analog audio and/orvideo signals may have alternative sources such as being generated by aVCR or CD-ROM player or received from a remote source via antenna orcable.

In a preferred embodiment, conferencing system 100 compresses anddecompresses video using the IRV method for purposes of videoconferencing. Those skilled in the art will understand that the IRVmethod of video compression and decompression is not limited to videoconferencing, and may be used for other applications and other systemsthat rely on or utilize compressed video.

In a preferred embodiment, conferencing system 100 compresses anddecompresses video using the IRV method. Those skilled in the art willunderstand that alternative conferencing systems within the scope of thepresent invention may use methods other than the IRV method forcompressing and decompressing video signals.

In a preferred embodiment, conferencing system 100 uses the IRV methodto compress and decompress a sequence of video images. In alternativeembodiments of the present invention, the IRV method may be used tocompress and/or decompress a single image either in a conferencingsystem or in some other application.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the principle andscope of the invention as expressed in the following claims.

What is claimed is:
 1. A conferencing system for a personal computerhaving a general-purpose host processor, comprising:(a) a first computerplug-in board; (b) a second computer plug-in board; (c) a videosubsystem adapted for residing partially in the host processor andpartially in one of the first and second computer plug-in boards; (d) anaudio subsystem adapted for residing partially in the host processor andpartially in one of the first and second computer plug-in boards; and(e) a communications subsystem adapted for residing partially in thehost processor and partially in one of the first and second computerplug-in boards, wherein:the video subsystem is adapted to:(1) receivelocal video signals; (2) compress the local video signals to generatelocal compressed video signals; (3) pass the local compressed videosignals to the communications subsystem; and (4) display the local videosignals in a first window on a digital computer monitor; the audiosubsystem is adapted to:(1) receive local audio signals; (2) compressthe local audio signals to generate local compressed audio signals; and(3) pass the local compressed audio signals to the communicationssubsystem; the communications subsystem is adapted to:(1) transmit thelocal compressed video signals and the local compressed audio signalsover a communications link to a remote computer system; (2) receiveremote compressed video signals and remote compressed audio signals overthe communications link from the remote computer system; (3) pass theremote compressed video signals to the video subsystem; and (4) pass theremote compressed audio signals to the audio subsystem; the videosubsystem is further adapted to decompress the remote compressed videosignals to generate remote decompressed video signals for localplayback; the video subsystem is further adapted to display the remotedecompressed video signals in a second window on the digital computermonitor simultaneously with the display of the local video signals inthe first window; the audio subsystem is further adapted to decompressthe remote compressed audio signals to generate remote decompressedaudio signals for local playback, wherein portions of the video, audio,and communications subsystem that reside on the host processor operateunder a window-based operating system running on the host processor; thefirst computer plug-in board is a video board; the second computerplug-in board is a combined audio and communications board; the videosubsystem is adapted for residing partially in the video board; theaudio subsystem is adapted for residing partially in the audio andcommunications board; the communications subsystem is adapted forresiding partially in the audio and communications board; the videosubsystem comprises:(1) a video microcode adapted for residing in thevideo board and for compressing video signals; (2) a video capturedriver adapted for residing in the host processor and for controllingthe compression of video signals by the video microcode; (3) a videomanager adapted for residing in the host processor and for controllingthe operations of the video subsystem; and (4) a video applicationprogramming interface adapted for residing in the host processor and forproviding an interface between an application adapted for residing inthe host processor and the video manager; the audio subsystemcomprises:(1) an audio task adapted for residing in the audio andcommunications board and for compressing and decompressing audiosignals; (2) an audio manager adapted for residing in the host processorand for controlling the operations of the audio subsystem; and (3) anaudio application programming interface adapted for residing in the hostprocessor and for providing an interface between the application and theaudio manager; and the communications subsystem comprises:(1) acommunications task adapted for residing in the audio and communicationsboard and for transmitting and receiving digitized signals over thecommunications link; (2) a communications manager adapted for residingin the host processor and for controlling the operations of thecommunications subsystem; and (3) a communications applicationprogramming interface adapted for residing in the host processor and forproviding an interface between the application and the communicationsmanager.
 2. The system of claim 1, wherein the portion of thecommunications subsystem adapted for residing in the audio andcommunications board is adapted to pass the remote compressed audiosignals directly to the portion of the audio subsystem adapted forresiding in the audio and communications board for decompression andplayback processing.
 3. The system of claim 1, wherein the portion ofthe audio subsystem adapted for residing in the audio and communicationsboard is adapted to pass the local compressed audio signals directly tothe portion of the communications subsystem adapted for residing in theaudio and communications board for transmission over the communicationslink.
 4. The system of claim 1, further comprising:(f) a conferencingapplication adapted for residing in the host processor and for providinga user interface; and (g) a conferencing applications programminginterface adapted for residing in the host processor and for providingan interface between the conferencing application and the video, audio,and communications subsystems.
 5. The system of claim 1, wherein theportion of the video subsystem adapted for residing in the hostprocessor is adapted to decompress the remote compressed video signals.6. A personal computer conferencing system, comprising:(a) ageneral-purpose host processor; (b) a first computer plug-in board; (c)a second computer plug-in board; (d) a video subsystem adapted forresiding partially in the host processor and partially in one of thefirst and second computer plug-in boards; (e) an audio subsystem adaptedfor residing partially in the host processor and partially in one of thefirst and second computer plug-in boards; and (f) a communicationssubsystem adapted for residing partially in the host processor andpartially in one of the first and second computer plug-in boards,wherein:the video subsystem is adapted to:(1) receive local videosignals; (2) compress the local video signals to generate localcompressed video signals; (3) pass the local compressed video signals tothe communications subsystem; and (4) display the local video signals ina first window on a digital computer monitor; the audio subsystem isadapted to:(1) receive local audio signals; (2) compress the local audiosignals to generate local compressed audio signals; and (3) pass thelocal compressed audio signals to the communications subsystem; thecommunications subsystem is adapted to: (1) transmit the localcompressed video signals and the local compressed audio signals over acommunications link to a remote computer system; (2) receive remotecompressed video signals and remote compressed audio signals over thecommunications link from the remote computer system; (3) pass the remotecompressed video signals to the video subsystem; and (4) pass the remotecompressed audio signals to the audio subsystem; the video subsystem isfurther adapted to decompress the remote compressed video signals togenerate remote decompressed video signals for local playback; the videosubsystem is further adapted to display the remote decompressed videosignals in a second window on the digital computer monitorsimultaneously with the display of the local video signals in the firstwindow; the audio subsystem is further adapted to decompress the remotecompressed audio signals to generate remote decompressed audio signalsfor local playback, wherein portions of the video, audio, andcommunications subsystem that reside on the host processor operate undera window-based operating system running on the host processor; the firstcomputer plug-in board is a video board; the second computer plug-inboard is a combined audio and communications board; the video subsystemis adapted for residing partially in the video board; the audiosubsystem is adapted for residing partially in the audio andcommunications board; the communications subsystem is adapted forresiding partially in the audio and communications board;the videosubsystem comprises:(1) a video microcode adapted for residing in thevideo board and for compressing video signals: (2) a video capturedriver adapted for residing in the host processor and for controllingthe compression of video signals by the video microcode; (3) a videomanager adapted for residing in the host processor and for controllingthe operations of the video subsystem; and (4) a video applicationprogramming interface adapted for residing in the host processor and forproviding an interface between an application adapted for residing inthe host processor and the video manager; the audio subsystemcomprises:(1) an audio task adapted for residing in the audio andcommunications board and for compressing and decompressing audiosignals; (2) an audio manager adapted for residing in the host processorand for controlling the operations of the audio subsystem; and (3) anaudio application programming interface adapted for residing in the hostprocessor and for providing an interface between the application and theaudio manager; and the communications subsystem comprises:(1) acommunications task adapted for residing in the audio and communicationsboard and for transmitting and receiving digitized signals over thecommunications link; (2) a communications manager adapted for residingin the host processor and for controlling the operations of thecommunications subsystem; and (3) a communications applicationprogramming interface adapted for residing in the host processor and forproviding an interface between the application and the communicationsmanager.
 7. The system of claim 6, wherein the portion of thecommunications subsystem adapted for residing in the audio andcommunications board is adapted to pass the remote compressed audiosignals directly to the portion of the audio subsystem adapted forresiding in the audio and communications board for decompression andplayback processing.
 8. The system of claim 6, wherein the portion ofthe audio subsystem adapted for residing in the audio and communicationsboard is adapted to pass the local compressed audio signals directly tothe portion of the communications subsystem adapted for residing in theaudio and communications board for transmission over the communicationslink.
 9. The system of claim 6, further comprising:(g) a conferencingapplication adapted for residing in the host processor and for providinga user interface; and (h) a conferencing applications programminginterface adapted for residing in the host processor and for providingan interface between the conferencing application and the video, audio,and communications subsystems.
 10. The system of claim 6, wherein theportion of the video subsystem adapted for residing in the hostprocessor is adapted to decompress the remote compressed video signals.